It's big, powerful and nasty. It's not humanoid, but familiar all the same.
This trope is for Eldritch Abominations that either naturally resemble (somewhat) ordinary animals, or else possess Animorphism and spend a good deal of time in the animal form.
The Beast, possibly the sentient incarnation of Gut's hatred and rage (and boy does hehave a lot of either) usually takes the form of a large canine with lightning-bolt eyes. The Berserker's armor, which previously had a Skull For A Helmet, is in the shape of a snarling wolf's head now that he wears it.
Several Apostles also have animalistic One-Winged Angel forms as well, most notably Zodd (pantherlike minotaur), Rosine (gigantic moth), and the Count (humongous slug).
In Animal Land, the Chimeras that Giller uses are unnatural monstrosities that aren't anything compared to the relatively natural looking animals of Animal Land. A number of them look more like they come from Berserk than a lighthearted animal manga.
In The Thing the monster spends most of its time as an Alaskan pack dog. You catch a glimpse of what it really is, before it proceeds to eat all the other dogs. It also appears very much like a spider/insect like organism in the prequel and in Carpenter's movie it still retains arthropod features like random insect legs. More horrifyingly, in the novel that inspired all of this, it is also heavily implied it assimilated an albatross, so it can now fly.
That said, there's no way of knowing what it really looks like, if it even has a "true" form at all, rather than being purely amorphous.
Mogget and the Disreputable Dog from the Old Kingdom trilogy. Respectively, Yrael and Kibeth the Bright Shiners.
WAAPT has ♨č▒⢫╠␣☣ł₤舸▟, or the Glitch Nidorino, a representation of the fear of death that merely takes the appearance of the first being it encounters in the material world, in this case being Vinollo the Nidorino. Luckily unlike most other Glitches it's friendly if its mission isn't to beat the fear of death into you.
Shadowrun has a lot of these in the form of magical beings like the totem Spider which plots world domination and manifests as a huge spider in astral space, and the Sega Genesis game has Thorn, a dog or coyote-like malevolent spirit.
RuneQuest. In the Glorantha campaign setting one major empire has an enslaved Chaos god/demon/thingy called the Crimson Bat, which looks something like a giant bat.
Two of the Elder Evils from Forgotten Realms—Dendar the Night Serpent and Kezef the Chaos Hound (based on the Midgard Serpent and Fenris Wolf from Norse Mythology).
Pathfinder's Rovagug bears more than a slight resemblance to a centipede, albeit one large enough to undo creation.
Many behemoths in Exalted resemble gigantic monstrous versions of animals, which are implied to have come first; there are references to "animals and the behemoths they were based off of".
Most of the Yozis resemble gigantic environments with many also having forms that resemble strange and transcendent humans, but Isidoros usually has the form of a monstrous black boar with innumerable tusks, great many-lobed eyes and thick barbed hairs, so large that his hoof prints are canyons and his back brushed against the sun, moon and stars.
This is also a possible fate for chimeric Lunar Exalted.
Phyrexia has a habit of churning these out, Green-aligned New Phyrexia in particular. Phyrexia is in the habit of swapping and recycling body parts between organisms, living or dead, organic or otherwise, with little regard for whether or not what they're making actually needs those parts. By way of example, the Rot Wolf; If you look closely◊, you can see that it does not contain any actual wolf.
In The Elder Scrolls series, several Daedric Princes have animal forms. In particular, Hircine appears in the form of a large white stag, and Clavicus Vile's conscience takes the form of a literalMorality Pet: a dog named Barbas.
Dragon Age: Origins features Witherfang, the giant wolf commanding a tribe of werewolves, who is presented to you as one of these at first. It turns out, however, that it is merely a wolf body possessed by the spirit of the forest it lives in—and one of the most human characters in the entire game.
The Beast in chapter one of The Witcher is a huge dog-shaped manifestation of the collective sins of an entire village, materialized to haunt and terrorize them.
A lot of the monsters from Silent Hill resemble animals. Dogs, cats, bird-bat things, roaches, and a crocodile have all been demonstrated.
From the same game, the Gohma also qualify. They resemble demonic versions of various animals with black skin and massive red veins all over their bodies and the smallest and weakest one of them is a gorilla.
The Liir of Sword of the Stars resemble dolphins or whales with a number of features from other sea creatures (fur, tentacles, negligible senescence...) and Psychic Powers, but most of them are simply Starfish Aliens. The Suul'ka, on the other hand are bigger than most starships, feed on the life-forces of planets, are summoned from the black depths of space where they slumber by mass sacrifices, and have a method of "teaching" their followers via direct telepathic contact that is almost always fatal for the follower. They're actually incredibly ancient Liir gone mad with power who've decided to take the Galactic Conqueror route.
Many of the later legendary Pokémon, especially Dialga, Palkia, Giratina (who personify time, space and anti-matter) and Reshiram, Zekrom and Kyurem (who were once a single dragon who may or may not have come from space).
Mewtwo could also count, even though he's a bit more human-shaped than the dragons.
Groudon, Kyogre, and Rayquaza really started the whole trend. Mewtwo was astoundingly powerful and dangerous, but was of entirely artificial origin, while Ho-oh and Lugia were, again, monstrously powerful, but while the former possesses abilities that certainly approached demigod territory, the latter was mostly just an exceedingly strong monster who stayed away from humanity. Groudon and Kyogre, on the other hand, are astoundingly old, slumber in places well beyond the reach of man, and can literally shape the world with their powers, and awakening them constitutes an apocalyptic event that could quite possibly wipe out all life on the planet. Rayquaza, meanwhile, is their mediator, which is to say that when those two have their squabbles, it swoops in to break it up and keep the planet from being flooded or reduced to a barren rock, and while it doesn't have the Walking Wasteland attributes of the other two, it's still frighteningly powerful and beyond the reach of man.
Almost everything with the "animal" tag qualifies as one of these.
Whateley Universe: way too many of the animal-like things in "Tennyo Goes to Hell", given where the story takes place.
Avatar: The Last Airbender has a couple of the spirits, including Koh, a giant centipede with a face that can change from human to animal to just about anything. And WanShiTong, a giant owl with an extendable neck.
The Legend of Korra continues this trend with the spirits of light and dark, Raava and Vaatu, which resemble gigantic flatworms.
Fantasia: the titular entity from "The Firebird Suite."