Trotty

Trotty is one of the two pig children who plays in the pool in Jolly Roger's Lagoon. As an unfortunate side effect of the toxic dumping from Grunty Industries, Trotty has grown a third arm on his left side.

Banjo-Tooie
Trotty claims it was due to his friend pushing him in the pool he emerged with a third arm. He says he's quite sensitive about it, to which Kazooie calls him "Freako". He also says he's feeling a third leg growing, though this could just be a slightly suggestive joke.

If you complete the sidequest where you heat up the pool first, he will thank you, but also add that "[he doesn't] want any more arms!" and Kazooie (and, curiously, Banjo) will say "Why not? Three more and you could be Spider Boy!"