I have mixed feelings about the suggestion. But mostly, I realize that this is unnecessary and, to some extent, a bit silly.
On the one hand, it could be neat for modders to be able to restrict their custom races to the use of certain mounts. Though, I doubt Toady would create a custom tag just for this purpose. It'd have to have a legitimate use, such as simplifying/streamlining the code for him or something. And if he were to add a new tag like this, it'd make a
whole lot more sense to define which mounts a race can or can't use in the race's raws,
not in the creature raws.
But, you see, Fortress Mode already does something like this. Take a look at the
Mount page on Magma Wiki. If elves use a mount they usually ride a unicorn. But they sometimes ride other stuff as, "It is theoretically possible for them to arrive riding any exotic land animal." Even so, there are only certain mounts that goblins ride and only certain mounts that humans ride. It's
already race specific.
However, one does not find a mention of such restrictions in the raws where one could modify such things. Another words, Toady must have this hard-coded somewhere other than the raws.
In my opinion, the "a bit silly" part is the suggestion that the use of a mount be restricted to certain biomes or regions. Are you suggesting, for example, that human siegers should not be able to ride camels unless the dwarf fort happens to be located in a desert? Or that humans should not be able to attack riding giant cheetahs unless the dwarves live in a jungle?
Personally, I find this matter quite trivial in comparison to the
real problem with mounts:
To this day players are unable to have their dwarves ride a mount, even though siegers can! Obviously, this has been suggested before. But it deserves repeating. Just recently I posted in the "DF Eternal Suggestion Voting" thread
here and
here about that.