I'm running my very own "shear anything that looks hairy enough" and "train anything vaguely canine" mods. I also have two modded creatures, which I created back in 40.19, that don't give me any problems whatsoever. I haven't touched any of the three body files. I might try replacing them with "fresh" files and see what that does.
I'm pretty sure it wasn't just the bison - replacing some of my other modded raws with fresh ones helped ameliorate the problem.
Errorlog has been pretty empty except for the oyster tallow crap I fixed fairly recently, and I had trusted that an empty errorlog meant nothing was wrong and everything was working as intended, but apparently that's not the case.
Since it happened with the fairy dragon as well, I've a hunch that the creatures themselves are not the problem.
And I know that if I remove the creature, the problem is fixed, but if I add the creature in any capacity, the problem returns. Replacing the three body files with fresh ones didn't help the problem one bit.
EDIT: Shifting its position a bit has allowed oysters and mussels to spawn normally, but apparently locked out pond turtles. Still, this is the closest to normal I've gotten so far.
RE-EDIT: Tacking on dingo raws to the bison just made the problem return. I might just go ahead and gen a world for the express purpose of having you guys take a look at it.