There is already the first matter to decide for the story. A herd from colder regions is comming to the Great Valley.
Do you think it should be a herd of members of only one species, or should it rather be a herd of different kinds?
The first variant makes it likely that the herd, while sticking together as a collective, is very racialist against other kinds; while they may be not as repelling against members of their own species.
The other variant makes it likely that the herd, while not being hostile towards any specific kind in particular, consists of very opportune members. It would probably provoke tensions within the herd as well and a kind of mood along the lines of: "everyone for oneself, don't care about the others".
Personally I'd prefer the second variant, on the one hand because it would provide us with more species and on the other hand for the personal reason that I have been dealing with a racist herd of threehorns in my current fanfiction already, so there is not too much new in this idea for me.