This is the underwater boss from Kirby. He swims around the level and spawns hooks, skulls, and mini Acros. The player can pick up the skulls and throw them at Acro to hurt him. By default, the boss dies after 3 hits.
Make sure to specify the sprite number of the hook, the mini Acro, and the skull in acro.asm! Check the ASM files for customization options.