From the Super Mario Wiki
Unithorns are purple-colored, rhinoceros-like enemies in Wario World. Unithorns are light purple, are rhino-like in appearance, and have huge thorns at the tips of their noses for horns. Their name, in addition, is a pun of unicorn, a legendary mythical horse with a horn, and "thorn". Only their floating heads are seen. They can be found only when Wario falls into 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.