Armothy is the name Morty Smith gave an arm which previously contained the life of an inhabitant from the Post-Apocalyptic Dimension. In his previous life, Armothy used to be a woodcutter in a small village, which was then pillaged by invaders. These invaders then burned his family alive and tortured him, causing him to seek revenge through murder. Armothy's fate, whether by the invaders or eventual death by the Death Stalkers, is unknown, but his arm eventually ended up hung out to dry in the back of a car, presumably fated to be eaten.
Armothy became a part of Morty when Rick Sanchez transferred Armothy's physique into Morty's arm to distract other inhabitants by fighting in the Blooddome, whilst attempting to steal a large deposit of Isotope-322. Though Morty was at first horrified, he soon used Armothy to vent his frustration toward his father Jerry for agreeing to his recent divorce from Beth. Eventually, Armothy found the leader of the invaders who ordered the attack on his village. Morty, realizing that once Armothy's revenge was carried out he would disappear, tried to get Armothy to delay revenge so Morty could continue to vent his frustrations. However, after deciding they both needed to move on, the two bid their goodbyes before Armothy choked the leader to death.