Pond Snail:

• Passive. Big & small variant.
• Big one has 30 hp, small one 15 hp.
• When hit, retracts into shell after a small delay? Stays in shell for x seconds.
• Drops Escargot Meat (very saturating). Small ones may drop a Pond Shell.
• Slow mob.
• Can thrive in both water and on land. Not affected by mud/quicksand.

Giant Leech:

• Swims in the swamp.
• Attaches to players, causing them to sink (even when they have Water Striders). While attached, the player also gets Slowness IV.
• Multiple can attach themselves to you. They attach themselves at point of impact and mold themselves (and their hitbox) to your body and the floor, if that makes sense.
• Sucks hp, deals damage every second while attached.
• When below 50% hp ( or a different percentage ), lets go of the player and tries to swim away.
• When far enough away, regenerates hp slowly.
• Drops Lifeblood & Slimeballs I suppose? Ideas appreciated.

Random musings:

We could eventually add a cave below the swamp that is filled to the brim with leeches. As an eventual reward, you could get a ''Leech Launcher'' that shoots smol leeches at enemies, slowing them down and dealing DoT while attached. These smol lads would fall off after like 5 seconds. Something like that would be a very cool reward. These leeches would also drag down flying mobs.