Beasts should be naturally evolving animals, not creatures that exist specifically on our earth. Because magic exists in D&D, there should be many beasts that aren't like other animals but count as beasts rather than monstrosities, just like how fish are massively different from mammals but still count as beasts. There are some cool ones, like dire wolves, which are beasts rather than monstrosities, but beyond that, we're throwing away some opportunities. And no, I don't believe owlbear should be a beast, since owlbears were (probably) created by wizard experimentation. What if something like a flying snake or axe beaks spawned naturally in the massively diverse and magical dnd universe and was a beast instead?

What are your thoughts? And would you like druids to have wild shapes that aren't generic animals? Besides switching stat blocks for some monstrosities, what new, unique animals would you like D&D to have?

I actually made a post about this earlier this year (sorry for the wall of text):

I don't get the Monstrosity creature type at all. There are some things in it that are obviously monstrous, like the dragonflesh grafters, but then there are things like rocs, which are just REALLY giant birds. I feel like a lot of the things in the Monstrosity type could just be made beasts, such as owlbears and even remorhazes. Why is a stirge, which looks like an amalgamation of bat and mosquito, a beast, but a purple worm, which is just a big worm, is a Monstrosity? There are so many animals in real life that are just as weird and creepy as stuff like carrion crawlers and ankhegs. Even the new dinosaurs from Bigby Presents: Glory of the Giants are Monstrosities, when all other dinosaurs are beasts. Or in Spelljammer, where asteroid spiders are monstrosities but all the space fish are beasts? It doesn't make sense to me, and it feels like it limits options for the polymorph spell and for moon druids. Does anyone have insight on why so many different things are Monstrosities?

Some of the responses talked about how 'monstrosity' could just refer to unnatural or abnormal creatures of any type, but I'm really of the same opinion as you. I feel like a ton of monstrosities could fit into the beast type just as easily, like some of the underdark creatures. Even owlbears would work, as the 2014 monster manual says the origins are up for debate. "venerable elves claim to have known these creatures for thousands of years, and some fey say they have always existed in the feywild."

I believe that limiting the amount of Beasts is exactly to make sure that druids don't get overpowered options or too many options for Wild shape - and I'm fine with that. 

Monstrosity is also the catch-all category of creature in DND; if it doesn't fit in any of the others, then it gets the monstrosity tag. 

"Monstrosity" (or "magical beast," as it was known in 3rd Edition) has always served as a dumping ground for monsters that don't clearly fit into other categories.