The ecology of the outer planes is often a topic of debate among planar scholars, such as how any ecosystem can function in planes of life and harmony where conflict is actively discouraged or in the hyper hostility of the lower planes. Accounts of how it might work are conflicted and varied as the bulk of study obviously goes to the resident outsiders and the often bizarre properties of these realms. What little is known about ‘mundane’ wildlife is the effect existing in such a place has on their physiology and psychology. In the case of the fiendish realms these creatures visibly change to a more nefarious resemblance from their material counterparts, darker colors, glowing red eyes, disturbing or macabre decorative patterns, and more devilish or demonic features in the cases of things like horns. Their hides toughen against extreme temperatures and even ‘prey’ creatures become profoundly more aggressive. While absolutely more willing to attack other creatures, even those that don’t present a threat, they seem to deliberately and instinctually react with immediate and focused hostility against good souls in particular. Fiendish animals, especially the auroch, are popular with the cult of Baphomet for all sorts of purposes.

Pathfinder's creature templates are real useful for providing variety to any 'themed' set of enemies, these guys were nice bricks to throw at the party during an encounter with the cult without it just being cultists and the like 2-3 demons in that CR range.

Art by my pal  Neon_Lizard