I think part of the reason animals are so infrequently used in RPGs is that they make for poor random encounters. Most animals flee when noticed, and if they attack they usually lack interesting things to do in combat. Because they are unintelligent, they make straightforward decisions. They do not speak your language.
But animals are great! Rather than acting as combat encounters, animals create environmental challenges. If players know how what a dog is like, they can interact with a dog in a meaningful way. A locked door can be picked or broken down. A guard dog can be killed, snuck past, bribed with sausage, taught the meaning of friendship, let loose to wander, etc.
Animal ecology can also create information about an area. Ants marching into a small hole might give a clue about a secret door. A bird in a dungeon hallway might suggest access to the surface. A bear implies its prey.
If a hostile area is primarily stocked with animals, you still want the players to have someone to talk to. Just let the animals talk; at least one. It's fun. It's almost a cheap trick. Nothing delights my players more than a talking animal. Is its personality stereotypical for its species? Hilarious. Is it against type? Still hilarious. Is it just a normal person? Somehow it's hilarious. You can't lose. Depending on the setting and genre, you can just let all the animals talk. If you want variety, I would use this:
d4 Animal Speech Capacity:
- no speech.
- Broken speech, a vocabulary of only a few words.
- Complete fluency in a language other than common. Draconic for lizards, thief's cant for raccoons, etc.
- Total fluency in common.
To demonstrate my thesis that animals are great, I will propose some scenes, details, and obstacles facilitated by animals in an adventure setting.
- A family of persistent beavers has flooded a section of the dungeon. tending a dam which threatens to disrupt an adjacent section.
- You desperately need some item at the far end of a bear cave. The titular bear is sleeping in the middle.
- A grove is covered in rotting apples, each riddled with burrowing wasps ready to respond to any movement. (This scene sponsored by my grandparents' old house.)
- A courtyard accesses several room in the dungeon, but is guarded by an aggressive rhino.
- Termites have rendered a section of a dungeon dangerously unstable.
- Dungeon denizens have trained cats to kill mice. They will go far out of their way to protect these cats in combat, and anywhere the cats cannot reach will be swarming with vermin.
- At dawn and dusk, bats flood the dungeon hallways, extinguishing lights and mucking up combats.
- Some crocodiles have learned to leave bait to trap their natural prey. For the dungeon croc, this bait includes coins, rolled-up scrolls, and potion vials.
- The password to enter the deeper dungeon is written on the underside of a wandering turtle.
- Wherever blood flows, swarms of flies are ready to cloud the room.
- Wherever bread is broken, swarms of locusts are ready to cloud the room.
- This underwater section opens to the ocean, and is lit by deadly men-o-war
- Since these dungeon denizens are immune to frog poison, dart frogs litter their living areas.
- Some maniac just threw a rattlesnake in a treasure chest and called it a day.
- a bee hive has formed in the mechanisms of this dungeon puzzle. Activating it will crush the hive and bring down their wrath.
- The moat is filled with completely mundane leeches, which will keep you from regaining health while undiscovered.
- Your foe stands confidently atop an elephant, which is plainly afraid.
- Hyenas follow you, waiting to feast on anyone you slay. They will alert dungeon denizens if bored.
- Climbing a chimney is frustrated by territorial cliff-nesting birds.
- You suspect a vital item was thrust into a weasel's den...