He and his gang can be found in the mining town of Redding after accepting the quest "Kill Frog Morton" from Sheriff Marion. "Frog" is stated to be a nickname that Morton gave himself, as "he makes people croak". His real name isn't stated. Sheriff Marion states that the terrible pun is enough to warrant his death.
Frog easily has the highest damage output, so it is advised he be killed first. Once Frog falls, his minions will die with relative ease. It should be noted that the fight can be made a bit more complicated due to many rats that infest Morton's warehouse. The player is advised to clear out the rats before accepting the quest, to make the fight a bit easier.