A is for Amoeba
Amoeba, leech and worm - no need to be giant to be challenging
Most of the time Amoeba, leech and worm are giant sized opponents of the characters. Those invertebrates live in wet, putrid locations explored by our adventurers. For instance in sewage, swamps, caves and lairs. An inspired game master can challenge his players by changing this cliché. Adding some warp taint is of course an option but other effects are also welcome. Here is a short table listing some attributes to use in game.
Curing capacities. From injecting or producing anti-toxin, anti-venom or any other curing effect. Can be used as a component for brewing the potion that could save an important NPC. Useful for going beyond the typical herbal component. The animal might need to be kept alive and brought back to civilization for the cure to work.
Elemental traits or capacities. From fire worms and their inner furnace, to some swamp electrical amoeba the range of customization is very large. Very useful as a deterrent to typical player thinking. For instance when players expect to found some Trolls behind digested remains, make them face some ground burrowing acid worms.
Enhance physical traits or capacities. Surprise character with levitating leech, wall climbing worms or sting throwing amoebas. Whatever else you think is appropriate to the encounter and its environment.
Magical effect. Pick one spell and allows the animal to cast it. From mind affecting effect like fear or more reality shaping like turning metal to vegetal. That’s an alternative to typical spell caster. Challenge players with an atypical source of spell. Give such creature to a non spellcaster handler, players will likely focus on the humanoid and not on the animal.
Disease carriers: bits and secretions from those animals can cause illness to characters. Pick one illness or an effect and use it. Good alternative to rats and insects.
Harmless traits: change something like the color of the animal. Animal has no other difference with a typical member of its specie. Players will take great care to manage those. Encourage cautious behavior.
Extend this table with your own ideas