This is a model of a Sea Spider, a fantasy-like creature swimming peacefully (or maybe not...!) around the Minecraft ocean.

Concept: This mob would spawn in ocean-like biomes, as well as in underground lakes, near the abandoned mineshafts. The Sea Spider would be neutral until the player decides to hit it. It then would try to attack (if the player was in the water at the time). But, if the player's out of reach for the Sea Spider, it would open its large mouth and spit out a poisonous substance, which would give Poison II for 10 seconds. After a successful hit, it would start moving slower and slower, until it collapses, similar to the bees that die when they sting something. Unlike Zombie Pigmen, er... I mean, Zombified Piglins, Sea Spiders don't group-attack the player and instead continue minding their own business.
Background: Sea Spiders are Squids fused with Spiders. The story behind that mob would be as absurd as the idea of it existing. A Cave Spider probably fell into the underground lake and was consumed by the Squid. That spider was though alive even inside it. So, after the Squid dehydrated (which they really like to do a lot), the Cave Spider continued living inside it and, slowly but surely, morphed into one creature. It then would spread across the whole Minecraft world and become one with this wonderful world's animals. ...Well, yeah, it's stupid, but I really tried.

Enjoy your new pal, squids!