This is a homebrew shape shifting demon, gaining strength to try and become a lord of a level of the abyss. All Eidolon type demons are shape shifter, but this one has taken the skill to a whole new level. What the reference shows is the creature's base form, of which an adventurer would basically never see before slaying the beast. Normally the demon is constantly shifting all of its limbs betwen different animal shapes to best suite what it needs for those couple seconds. So maybe while sleeping it rests in a tree with bird legs and lion arms, then uses sloth hands to swing down, a monkey tail to stabilize, then shifts some legs and arms to giant spider legs to catch itself on the floor, before finally shifting into polar bear with a wolf head and assorted limbs to find what woke it.

The demon is a master hunter and wants nothing more than to track and take down progressively stronger and stronger prey until there is nothing left. He also has a huge soft spot for small babies, often particularly targeting parents so he can kill them and steal the children for his own. However, the demon quickly gets uninterested with them as they grow so they either die from niglect or grow vicious and follow the demon in his growing wild pack. These animals will sometimes aid in the hunt, but usually just tail to catch stragglers and feed on the demon's scraps.

I may use this demon as a pc with the additional backstory: The demon was going to be summoned by an evil cult, though a beyond noble palidin tried to stop them. Despite freeing the sacrifices, the paladin was captured and used as a vescle to transform into the essence of the Eidolon. However, the paladin's physically binding oath blessing stayed with the form even as it was permanently transformed. The demon quickly found that it would be resteained by magic light chains if it attempted to do anything chaotic or evil, and as he struggled the chains quickly stole most of his powers to transform. Through quite a bit of trial and error (and some situations where helping someone was personally benifitial), the demon found that great acts of good would convince the chain to give back some of his capabilities. The demon feared trying to remove the chains with anything less powerful than a wish, since they could easily force him back to the abyss instead. Or worse, they could force him out of the body and yet leave him in pain from the binds of the chains for all eternity. So now the demon simply wants to find the most violent and hunting friendly method possible to convince the chains to free his powers. At least until he can wish them away.

To play youd reflavor a Firbolg as this demon (especially useful since he maintains some base powers like superior disguise self, detect magic, and short term invisibility). Then you build a multiclass barbarian/moon druid, reflavoring each level up as some of the demon's original powers returning.