From the Super Mario Wiki
Unithorns are purple-colored, rhinoceros-like enemies in Wario World. They do not appear to have bodies; only their floating heads are seen. They can only be found when Wario falls off a level (down a pit) and lands in the Unithorn's Lair. They will try to bite Wario and steal his coins. To escape, Wario has to break various boxes to find a single one with an Escape Spring inside. All other boxes have Smiley Bombs.
Unithorns have a fear of Smiley Bombs as they are stunned for a fraction of a second when one is uncovered. They will be stunned again once the bomb explodes.
Unithorns are light purple in color, are rhino-like in appearance, and have huge thorns at the tips of their noses for horns.