Quote from schm0 >>

Do gunpowder, firearms and cannons belong? Artificers? Futuristic weapons or technology? Flying ships? Space ships?

 For many, the answer is no. Official WOTC content notwithstanding, some people like their fantasy to feel medieval and Tolkien-esque. Anything more "modern" than that, as another poster pointed out, typically gets classified as science fiction - and while there are plenty of folks with crossover genre interests, that isn't true for everyone. 

I still say that if having lasers and dinos and fish-people from space coexisting with dragons and wizards and castles is fun for you, go nuts. People with the same preferences will gravitate to your table, and people with diverging preferences will find another. 

The question is "do they belong in fantasy", not "do they belong in your setting".

I think it is absolutely valid for these things to exist in a fantasy setting, just maybe not yours.

Do people have the same issue when seeing like a Mammoth or a Saber-Toothed Tiger? Dinosaurs are animals just like any other, but they have the added benefit of being cool enough to stand in as some interesting monster or domesticated animal options in a fantasy setting instead of your traditional large beasts like Elephant or Rhinoceros.

So, yes, I'm of the opinion that dinos are good for fantasy. Whether you choose to use them is up to you.

From a writing perspective, it goes like this:

Horror lets you explore the idea that normal human beings are not Apex Predators.  Sure, we personally fought Dire Wolves, Short Nosed Bears, Giant Sloths, Mammoths etc. etc.  Sure they are all dead and we live. Sure we are terrifying creatures red in tooth and artificial claws, but I FEEL so weak and helpless that there must be creatures out there that I have no reasonable chance of surviving. I wanna crawl into a fetal position, not eat a cut of meat from something that weighs 5x my weight and has huge horns on it's head.

Science fiction lets you explore concepts outside of reality and most importantly lets you make up your own rules so you do not have to be knowledgeable of effective tactics and techniques to right in a 'realistic' manner.  You want melee combat to be better than ranged? Make yourself some kind off vibro knife/light saber.  This obviously includes Horror, which is a concept out of reality. Horror is just a limited version of Science Fiction where the only rules we break are the ones about humans being deadly killing machines.

Fantasy lets you go further and explore any possible concept totally ignoring even obvious and simple rules of reality.  Lets raise the dead, walk on water, make wishes!  This clearly includes all Science Fiction.  Science Fiction is really just a limited section of Fantasy where we try to keep it more realistic.  Just look at Star Wars which they swear does not have wizards capable of magic, its the Force, not magic. Note, Fantasy has the least rules and therefore is the 'easiest' to write without people saying 'that is not real'.

Yes Dinosaurs are science fiction.  They are also Fantasy. Also note, people in Fantasy books can still have guns (see gunslinger).  Just because we ignore some rules does not mean we cannot keep some features of reality.  There are lots of Fantasy books set in modern day. These categories are inclusive of earlier categories, not exclusive

This is a no-brainer to me because dinosaurs are an option for your campaign. If you don't want them, don't include them. But if anyone wants them, then they should be in. Cause then everyone can be happy.

I have had campaigns where dinosaurs fit and campaigns where they didn't. I have at times reflavored them to something similar, but different enough to avoid all the "baggage" referenced above. D&D's scope is much wider than what most people would consider to be "proper" fantasy, and fantasy itself is big and varied enough that anything can fit in it when put in the right context. The game needs a wide variety of content to support that.

Dinosaurs have been in D&D for many if not all editions.  While not necc typical in "high fantasy" faire, they do show up in some "low" fantasy as well as swords and sorcery fantasy. Dinosaurs, or a dinosaur, show up in Conan.  It's not an uncommon idea in fantasy to have a sort of "savage world" or "savage lands" that populated with dinosaurs that don't make irl geological sense to be amongst each other, let alone Homo sapiens, but it's fantasy because dinosaurs often provoke the imagination, and "wouldn't it be cool to fight one or ride one or hunt one" is common landing of that imagination.

This is to say nothing of dinotopias, where dinosaurs coexist and are integrated into a human civilization (Bam Bam was a 4th level Barbarian, fight me) or and more "plausibly" integrated into some sort of evolved "saurian" civilization (like some worlds do with their Lizardfolk).

Most D&D worlds were created by gods or other powerful primordial beings thousands or tens of thousands of years before the time period the game is set in, with the species on them either being also created by gods or being brought to the world from another world via magic.  They're not billions of years old with species diversifying through evolution.

Also, I notice that these discussions inevitably center on dinosaurs only while ignoring the game's other anachronistic-for-Medieval-Europe species like dire wolves, sabertoothed "tigers", mammoths, or cave bears.  Or the fact that settings like the Sword Coast that are based on Medieval Europe still have corn, potatoes, tobacco, black bears, mountain goats, raccoons, and various other things that were definitely not present in Europe at that time.

I was referring to the fact that dinosuars "Feel wrong" / break immersion / are dissonent, as Solarsyphon put it. It's fine if you personally don't mind having it in your game, just as it's fine if you want to have spaceships and martians in your game. I was trying to ask, would you personally find dinosaurs annoying if your DM used them, or would you avoid using them if you were/are the DM? Also, if you find them undesirable, is there anything you would do to make them less undesirable?

I feel like dinosaurs do belong in some DND campaigns. It could be a campaign with time traveling mechanics, so the DM might add in a dinosaurs or two.

Either way, I've been a player of a good amount of campaigns, and the DM of others. In a player's perspective, it is really fun to have a dinosaur to either fight or try to befriend.

But be warned, if your campaign has someone who has a animal shifting component in it, I believe there is an animal sheet that if they see an animal, and their something level is high enough, they can change into the animal.

P.S. I believe that druids have this ability.

It's really going to depend on the setting to my mind. 

In my worlds they are loosely analogous to 800-1100AD Europe if they are high fantasy. That means that guns and firearms do not exist and never will. Likewise Artificers and Constructs just don't fit thematically. That however is my subjective opinion. I can also see how that wouldn't be true in someone else's world.

Dinosaurs feel aesthetically 'off' in my current world setting, so I have taken the Arthur Conan Doyle approach and isolated dinosaurs to an island continent that is either unexplored or not often travelled to. They exist, but you're not going to encountering them on every plain or continent of my world. 

Again though, consider the Terry Pratchett approach. His commentary on the fantasy of Discworld is that every profession or era was set in this nostalgic 'golden age' where it was at it's best. So you get medieval town watch, victorian sweet shops and department stores, you get covens of witches, edwardian governesses, roaming heroic barbarians, and tourists from far away lands with fantasy cameras. A hodge-podge of history is entirely valid as an approach and can work. 

It's about what works in that particular setting. If it's been set up as a cherry picking of fantasy tropes, then that's cool...it's what works for the world.

Quote from AEDorsay >>

I doubt Strahd has much of an issue dealing with Dinosaurs. 

 we need dinosaur domains of dread! NOW!

     I haven’t read all of the comments yet, so pardon any redundancies.                                                                                       I think it comes down to the setting. I am going to use MtG as an example if you have any experience in that game. If not, then think of the D&D settings of Strixhaven, Theros, and Ravnica. These are all from MtG. The MtG also contains a world called Ixalan. It has “Spanish Conquistador” and “Jungle” themes. Dinosaurs seem to fit there. If we go back to Ravnica, however, there seems to be no reason for them to be present. 

I personally think you can put dinosaurs in you just need to harmonize them a little bit in some cases. As some examples: 

If you take LOTR for an example. Their history is one where  divine beings have been battling for millennia with ancient super civilizations and dwindling magical power.  To fit a dinosaur into that it just needs to fit into that divine war.  An example would be making dinosaurs a corruption of existing animals by morgoth from an ancient time and adopt an appropriate skin. Whether it be something like the melted aesthetic of the movie orcs or a black sleek design like the black wings their appearance will be better if it tells the story of what they are in the setting as opposed to just copying the real world appearance.  

 Another common example is your classic druid. Many druids have aesthetics and story related to ideas with modern European style forests. Bear skin cloaks, pet squirrels and a harmonious view of nature. A big lumbering t rex doesn't really fit in that environment but its not hard to choose dinosaurs that do or adjust them to do so. Make them smaller, use green colours or use aesthetics on your druid that the dinosaurs you want. I similarly don't really like moon druids turning into elementals because I don't think the moon druid has any real elemental theming outside of this ability and it takes over the class at high levels because they're mechanically the best option. Dinosaurs can be similar in that often players adopt them mechanically at the cost of a coherent character.

 Faerun harmonizes dinosaurs by creating a separate continent with a god which created the dinosaurs to act as his servants so they get revered as divine beasts. Dragon still feel kind of weird in faerun with dinosaurs because they're so similar to dinosaurs and are described as dinosaurs kind of inconsistently on wikis ect.. but you can fix that problem by making dragons there go either way, either being dinosaur servants of ubtao or foreign invades by foreign gods like tiamat. 

Quote from Capatin_Crunch >>

Quote from AEDorsay >>

I doubt Strahd has much of an issue dealing with Dinosaurs. 

 we need dinosaur domains of dread! NOW!

 there is Valachan in van richtens guide. It's sort of a predator style setting with dinosaurs.

Quote from Solarsyphon >>

Quote from Capatin_Crunch >>

Quote from AEDorsay >>

I doubt Strahd has much of an issue dealing with Dinosaurs. 

 we need dinosaur domains of dread! NOW!

 there is Valachan in van richtens guide. It's sort of a predator style setting with dinosaurs.

 you remember Cyre 1313? DINOSAUR TRAIN!