So cavaliers and some archetypes get a "Mount." Now it says anything suitable can be their mount. Is there a way to tell if a mount would be suitable? Or are they pretty much stuck with the 1 of the 6 listed animals?
For Society yes if not ask your GM(if you are the GM you can decide)
Mechanically, mounts have to be one size category larger than the rider, strong enough and physically able to carry them around, and capable of being trained.
My understanding is that halflings use riding dogs at least as much as they use ponies. Dwarves don't seem to have a strong tradition of cavalry, though they use ponies for carrying things and riding if necessary. Elves use horses, though I've seen them ride large deer and elk in some settings. Gnomes don't seem to have any traditional mounts in Golarion. Goblinoids of various kinds frequently use wolves or worgs (evil talking wolves borrowed from Tolkien).
There's also axebeaks (essentially chocobos or riding birds), various dinosaurs if you're into that, and maybe some lizards that would make suitable mounts.
You can also ride some varieties of giant vermin, but that usually takes some extra class features to be able to train them.