Worldbuilding: Fantastic Beasts and Where To Find Them

As an old school D&D person, one of the things I loved about the old Monster Manual was that it wasn't just a listing of monsters and their combat stats.  Care was taken it A. also putting regular animals in there and B. discussing the behavior, habitats, biology and ecology of all the creatures discussed.

Earlier when I talked about Bottom-Up Worldbuilding, I touched briefly on adding the flora and fauna, and how those decisions would shape the cultures around them.  Of course, those choices also have to fit the environments around them.  You can't just, for example, stick lions or tigers or bears just anywhere because they're awesome.

Nor can you have any animals just be domesticated, just because that's what you want.  The animals domesticated over the course of human history were domesticatible for a reason.  And more to the point, those that were NOT, were not for a reason.  Zebras are the angry jerks of the equine family, and they do not want to be your friend.  Bison are skittish and can JUMP like you wouldn't believe, and thus are challenging to put into a corral.  Elephants can be tamed, but they stay pregnant for two years, just to have one calf, which makes raising them for meat or transport unideal. 

But this is fantasy worldbuilding, and you can make whatever creatures you want, right?  Right. But make sure they make ecological sense. Fast-moving apex-predators, for example, are going to need a lot of calories, so their food supply should be ample, and there's a limit to how much competition they should have for that supply.   Non-domesticatible megafauna should have a good reason for not having been hunted to extinction for their delicious meat.*

And if you want to create a from-whole-cloth domesticated animals, remember that they should include many or all of the following traits:

1. Flexible diet (i.e., it can eat a variety of things, especially the foods that humans can either spare or not need themselves)
2. Reasonably fast growth rate (i.e., one where it's worth the trouble to domesticate it-- if it takes several years of care before you have a useable animal, it's not worth it.)
3. Can breed in captivity 
4. Pleasant disposition and minimal/usable panic reflex.
5. Social hierarchy, especially ones where humans can take over the role of pack leader.
6. Useful resource for humans: including and not limited to meat, milk, cloth, transport or labor. 

What animals do you use in your worldbuilding, and how do humans interact with them?

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*- Australia had quite a bit of mega-fauna**, including cow-sized marsupials and sheep-sized monotremes (platypuses), but a leading theory is that when early humans arrived in Australia, they were already accomplished hunters, while the Australian animals were not accustomed to humans or being hunted, and thus were easily hunted.  Contrast that to African megafauna, which had the advantage of evolving with humans, and thus learning how to adapt to human hunting techniques.
**- If you want some inspiration for fantastic-seeming creatures, googling "Australian Megafauna" is a wonderful way to go.