Trotty

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

Banjo-Tooie
Trotty claims it was due to his friend Piggles pushing him in the pool that 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!"

Trivia

 * Third leg is a slang word for penis, In contrast to the E-Rating. Trotty is really talking about an extra leg.
 * When Kazooie said You could be Spider boy to Trotty this could be a reference to Spider Man.