Pokeys are enemies you encountered in previous games, but they were mostly found in desert-themed levels. While those are not in 3D World, the Snow Pokey variation is in the game, which as you guessed by the named, is in snow-themed levels. They function pretty similarly to Pokeys, as they are made up of segments, all of which have spikes on them. That means touching any of the body parts will cause damage to the player. They also come in varying heights, so while one Snow Pokey might have two body parts, another might have four parts.
Of course, all of them will have a head, which is their weak point. If you jump on top of their head, then the whole Snow Pokey will be defeated. You can take out body parts one by one with a power-up, leaving nothing but the head, with the parts turning into snowballs. The snowballs can be picked up and tossed at other enemies, or used to collect some collectibles. Note that if you use a Fire Flower on the body parts, they will not turn into snowballs.