From the Super Mario Wiki
Chikunto (チクント) are subspecies of Ant enemies, but are able to protrude spikes from their bodies. If Mario jumps on them when they do this, he will gain damage. They only appear in the Macro Zone of Super Mario Land 2: 6 Golden Coins.
Usually, they will walk with their spikes retracted, and then stop to stick out their spikes as Mario gets near. Mario must wait before he can jump on them. They are defenseless to a fireball, regardless of whether or not their spikes are protruding.