Just like Avatars, but as a second type of “religion” and less costly.
The way of implementation can be anyway…warpaint can be a good way, can only learn 1 at a time, and maybe give it a cool down and a trigger
There can be all sorts of creatures, for example:
Ape for great melee combat
Shaleback for great defense
Croc for swimming and underwater
Crow for flying&scouting (can do hardly any damage)
Bat-demon for gliding from higher place to lower (A magnificent falling without fall damage, kind of…)
Rhino for carry stuff
wolf/hyena get great bonus when travel in pact.
Cobra for hiding and hitting, works extremely well in grassland
Rocknose can travel as pile of rock and hide underground, works well in rocky area
Same thing, Scopion for desert…
Explosive imp for suicidal bomb, note that warpaint is not cheap and you lose warpaint and your stuff doing so
Jaguar/leopard for great plain ground travelling
etc. With mounts as not a possibility, shapeshifting is the best way for ppl to travel, and offers many ways of fighting style especially in pvp