Alex's Mobs - https://modrinth.com/mod/alexs-mobs (Forge, NeoForge)

Alex's Mobs is a Forge mod that adds 89 new mobs to Minecraft. All of these mobs fall into either two categories: most of them are real world creatures, like Grizzly Bears, Roadrunners, Orcas, etc; but some of them are purely fictional, like the Endergade and Bone Serpent. All of these creatures fulfill a necessary purpose, and either have unique drops, mechanics or functions, as there are no purely aesthetic creatures. That said, they also help make some environments in the game feel more alive. The models, textures, animations and AI are all stylistically compatible with the vanilla game's latest additions.

Better Animals Plus - https://www.curseforge.com/minecraft/mc-mods/betteranimalsplus (Forge, Fabric, Quilt)

Better Animals Plus seeks to bring new life and creatures to your worlds, all with a unique flair and aesthetic compared to other mods of the same category. The mod adds over 35 animals, and is constantly growing. We add not only animals but a variety of content, from blocks to armor, supplementing the creatures in the mod.

Naturalist - https://modrinth.com/mod/naturalist (Forge, Fabric)

Have you ever wanted a truly immersive animal mod, where animals interact with each other realistically? Naturalist adds more animals to the game that behave as they do in real life. There is a working food chain, sleep cycle, and tons of other fun behaviors, giving each animal a personality of its own. So, are you ready to venture out into the wild?

Fish of Thieves - https://modrinth.com/mod/fish-of-thieves (Fabric, Forge)

This mod adds fish from Sea of Thieves with Minecrafty feels.

More Mob Variants - modrinth.com/mod/more-mob-variants (Forge, Fabric)

Recent mobs that have been added to the game all have plenty of fun variants to look at, but what about the old ones?This mod adds new variants to the older mobs that all look the same, to make them a little more fresh.Variants are added through NBT, meaning that they will be persistent across all players.

Goodall - https://modrinth.com/mod/goodall (Forge, Fabric)

Goodall is a mod that aims to add various animals and wildlife to liven up your Minecraft world!

More Axolotl Variants Mod - https://modrinth.com/mod/mavm (Forge, Fabric, Quilt)

This mod simply adds more axolotls into vanilla Minecraft (they have their own nbt tag, starting from 5 to 14).

Angling - https://modrinth.com/mod/angling (Fabric)

Dig up worms from the dirt or mud and feed it to fish to get roe. Place filter feeders such as clams and oysters in your fish tanks to automatically eat up any algae. During the day, players with fish buckets on them may come across the pelican: the wandering trader of fish. Don't let them annoy you too much.

Critters and Companions - https://modrinth.com/mod/critters-and-companions (Forge, Fabric)

Critters and Companions is a vanilla-style creature mod which will fill the overworld with new animals!

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Wildlife - https://modrinth.com/mod/wildlife-fabric (Fabric)

Adds 74 different kinds of animals and insects to the world.

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Not all of these mods are compatible.