D&D doesn't even make a pretense of trying to follow modern taxonomy.  Most humanoid races were created via the direct actions of a god or other powerful mystical being.

Also, laying eggs does not preclude feeding a young with milk.

Wait does this count as thread necromancy? There was a post 1 day ago but before that the last post was from 2017. Sorry I don't really understand the rules.

I don't think it does, because it's not a question specifically for the thread creator.

Technically all mammals come from eggs to.  Ask any gynecologist.   The difference is mammals hatch from the egg inside the womb and then are birthed.  

Although every once in a while you get a mammal that does not quite manage to break through the egg.  This one could be described as hatching after the birth:

https://www.foxnews.com/health/baby-born-intact-amniotic-sac

I think that what people mean by "eggs" on this thread they mean "eggs that hatch into babies outside the womb" or "hardshelled eggs that the babies hatch from outside the womb". Yes, humans have eggs, but they are very fundamentally different from any reptile, bird, or fish egg. 

Echidnas and Platapus have hardshelled eggs, and do feed their young with mammary glands, it kind of "sweats" off of them. 

So, I would say, generally, Aarakocra are birds, Kenku are birds, Lizardfolk, Dragons, Dragonborn, Tortles, and Kobolds are reptiles, and Tritons, Merfolk, Locathah, and Kuo-Toa are fish, and Grung are amphibians. 

Pretty much anything else that is a humanoid would count as a mammal. (I have no idea what Koalinths are)

Koalinths are a seaborn goblin, who walk faster than they swim,  so I would say they are mammals.   Tortles are obviously reptiles.  

Quote from Mog_Dracov >>

Koalinths are a seaborn goblin, who walk faster than they swim,  so I would say they are mammals.  

 But they have scales, and live in the ocean. This is a similar circumstance to Merfolk or Tritons. 

And how do you account for things like merfolk being cross-fertile with humans?

Quote from 6thLyranGuard >>

And how do you account for things like merfolk being cross-fertile with humans?

 Can humans breed with merfolk? 

(A lot of large sea creatures through Earth history give live birth to young, like Ichthyosaurs, Mososaurs, Sharks, and many others. I don't know if Merfolk lay eggs or not.)

I would say that humans and merfolk could have children. But that's up to the DM to decide (if you somehow got in a scenario were you needed that information).

Half-merfolk were a PC race in previous editions.

My point is, you can't really classify half and half organisms in D&D into neat modern taxonomic categories because they very explicitly have all kinds of different oddball effects going on and there's all kinds of weird magical interactions.  And that's not counting cases like Githyanki, which were original humans enslaved by Mindflayers and were experimented on until they became the original Gith race, who rebelled and then split into two factions that eventually became the Githzerai and Githyanki, who then crossed their blood with red dragons so much that in 2nd and 3rd edition they layed eggs.  Or the Sahaugin, who are humanoid sharks, except that sometimes their young are born looking exactly like sea elves.  Or Driders, which are simultaneously vertebrates (dark elves) and spiders.

Quote from Joshua_the_Autistic_Tiefling >>

It makes sense for aarokrocra and kenku to lay eggs. However, I doubt dragonborn do because most female dragonborn are depicted as having breasts.

 There are many instances in pop culture and literature of Dragons laying eggs but also nursing their young. Look at Wyverns from Ark for example.

Quote from FuzorFishbug >>

Quote from DungeonLord58 >>

Quote from Purpzie >>

After all, mammals don't grow feathers.

 True. The feature that defines avian creatures from all other animals is that they have feathers.

 Oh jeez, I can hear Diogenes of Sinope coming and he sounds mad.

 FEATHERLESS BIPED? THIS IS A MAN

Quote from IamSposta >>

Quote from Joshua_the_Autistic_Tiefling >>

It makes sense for aarokrocra and kenku to lay eggs. However, I doubt dragonborn do because most female dragonborn are depicted as having breasts.

 There are many instances in pop culture and literature of Dragons laying eggs but also nursing their young. Look at Wyverns from Ark for example.

 to be fair, the Wyverns in ARK are also genetically modified abominations made by Scorched Earth's AI and is in no way actually representative of dragonkin as a whole :P

Quote from DevanAvalon >>

Quote from IamSposta >>

Quote from Joshua_the_Autistic_Tiefling >>

It makes sense for aarokrocra and kenku to lay eggs. However, I doubt dragonborn do because most female dragonborn are depicted as having breasts.

 There are many instances in pop culture and literature of Dragons laying eggs but also nursing their young. Look at Wyverns from Ark for example.

 to be fair, the Wyverns in ARK are also genetically modified abominations made by Scorched Earth's AI and is in no way actually representative of dragonkin as a whole :P

 Never claimed they did. Just pointed out one example of a precedent if any DMs wish to rule it that way. 

I can point to The Dragon Nimbus novels (or maybe one of the related series) as an example of Dragon breeding. Where a female dragon tried to attract as many “fathers” for a mating as possible in a mid-air snake breading ball for the healthiest clutch possible. My point is, it is however the DM imagines it to be.