This is an iron golem textured in the Super Mario Mash-Up Pack (https://minecraft.fandom.com/wiki/Mash-up_pack#Super_Mario_Edition). It's supposed to look like Megahammer. Megahammer is a boss from Super Mario Galaxy 2, but that's pretty much all I know about it.

"An iron golem is a large strong neutral utility mob that defends players and villagers. It is one of the few mobs that can be built then spawned in the game. " -https://minecraft.fandom.com/wiki/Iron_Golem