From the Super Mario Wiki
Snailicorns are large snail-like creatures that have the characteristics of a unicorn, such as a large horn, hence the name, which is a portmanteau of snail and unicorn. They appear only in New Super Mario Bros. and are never seen outside of ice stages. They can be very dangerous, as a mere jump is not enough to defeat them, and fireballs are ineffective. However, as jumping on them sends them back a few feet, this tactic can be used to send them off of a cliff nearby, though they'll reappear if Mario leaves the area and comes back. Also, a full-speed Shell Mario or a stray shell will knock them out.
They always charge in a straight line, but change direction at edges, similar to Red Koopa Troopas. They only appear in World 5-3 and World 5-B.