Seahorse

They drift around the ocean floor in short random darts, which makes chasing them with a Bug Net more fiddly than it looks. They also have almost no health, so a stray shot from you or a hit from anything else in the water kills the thing you were trying to catch. Clear the local enemies first, then swing.

A small fraction of spawns come up gold instead. Gold critters sell for a lot and make the best display pieces in a base, so it is worth a second look at every seahorse rather than netting the first one you see and leaving.

Turning it into a Seahorse Cage locks the critter in permanently. If you later want the seahorse itself back, Shimmer is the only way to undo it, so keep a spare in a chest if you are the sort who redecorates.