He was unhappy about being pink, when he loved so many green things. Lucky for him that a Green Monster was having a similar problem, so they ended up switching colors. However being a new color did not solve all the problems so they decided to switch back. Puppyduck then gave the Pink Monster and the Green Monster, green and pink spots so they could get the best of each color.
The Pink Monster was originally part of The Fazoobs, it was used in various roles on Mopatop's Shop.