It may not be a mutant per-se, but it sure is an evolutionary rarity. From Reuters:
The frog, Amolops tormotus, is the first non-mammalian species known to use the ultra-high frequencies that humans cannot hear.
It comes in handy to be heard above the pounding waterfalls and streams in the mountainous region of east-central China where Amolops tormotus, which is known as the concave-eared torrent frog, lives.