As I stated in the introduction, certain mounts and companions are reserved for certain races or specializations.
Below is the needed information to make an informed decision on your mount or companion.
Horses -- Free for all, except for war horses (reserved for noble and warriors)
Large Wolves and Bears -- Reserved for ranger specialization
Halla -- Reserved for Dalish elves
Wargs (mean, kinda nasty looking large wolves) -- Reserved for rogue class
Mabari Warhound -- Reserved for nobility and warriors
Drakes -- Reserved for Mages