A spiked creature dropped from above by Lakitu. Its spiky shell makes it impossible to stomp — jumping on one hurts Mario instead. Spinies walk in one direction after landing and cannot be stomped under any circumstances.
Hit with a fireball, or knock a Koopa shell into it. You absolutely cannot jump on or stomp a Spiny — its spikes will hurt you. The best long-term solution is to defeat Lakitu before he drops too many.