"Mosquito fish" is a common name used to refer to several small American Gambusia species, sometime introduced to ponds to control mosquito larvae. These three are Gambusia affisni holborooki.
The male, Spot, has his distinctive piebald coloration because he has melanistic spotting. This is a naturally occurring Y-linked mutation, meaning it only affects males. It also doesn't breed true, so some offspring of melanistic males are gray colored.