Mack Salmon is from the Sam and Max, Freelance Police franchise and one of Sam and Max's arch enemies.
He first appeared in Night of the Gilded Heron-Shark, a comic book.
He first appeared in the television series in the episode, "They Came from Down There", where he tried to used innocent people to make Sea monkey related merchandise and meat by-products by stealing their brains with a mutant alligator gar fish. He was defeated by Sam and Max because the creature failed to steal Max's brain and unintentionally destroyed the lab in the process. Salmon was later faced with the unpleasant fate of being personally dealt with by Max.
In the show's final episode, appropriately named Final Episode, Salmon teams up with other Sam and Max villains in an effort to destroy the heroic duo for interfering with their plans. Sam and Max are successfully captured and tied to a bomb in a plane, which the villains plan to drop into a volcano. This will will not only take out our heroes but the rest of the world, should it explode. Sam and Max escape, taking ample time for flashbacks to the series past, only to be recaptured. The Villains are finally defeated after the sudden arrival of the Rubber Pants Commandos with Mach Salmon being cast into the volcano.
Because the show ended with only one season it is not known if Mack Salmon survived his volcanic encounter, but it seems unlikely. Because, well, who survives a volcano? Certainly not a fish.
Appearing to be a human with a fishbowl for a head, Mack Salmon is really a super intelligent fish who travels on top of a fake body. He blames Sam and Max for causing his current form through some past incident (of which they have no memory) and is perpetually seeking revenge against them. He likes to revel in self-importance and see himself as their arch-nemesis, but in reality he is little more than just one of a variety of bizarre enemies they've come across. In Night of the Gilded Heron-Shark he and two goons confront Sam & Max simply by waiting for them in the office, but are thwarted by the timely appearance of The Rubber Pants Commandos. In the animated series he returns as a more typical evil mastermind using some more elaborate plans, and is probably the most competent enemy they face.