I second everything MoonMan-kun suggested and Isekai Ojisan. Might as well have been a monster
If monsterous as in acts like a monster/villian is acceptable,
Saga of Tanya the Evil's anime adaption was very enjoyable at the least.
Unfortunately I do not have a recommendation for a monster who acts monstrous (other than overlord, which is kindof half monster/dead inside, half human cringing at his own life)....
As a lesser known/lower quality suggestion,
Skeleton Soldier Couldn't Protect the Dungeon. He isn't op at first, but his cheat skill guarantees he will become op so might as well be. He is like subaru from re:zero,
+evolutions
−skin.