Isaac was a part of the Garrahan family who ran Garrahan Mining. Unlinke the other members of the family he didn't devote a great deal of his time to the company, instead spending time with his pet sloths. He went so far as to call himself their savior and master. This was not well accepted by Isaac's brother, Bill Garrahan, who threatened to kill any sloth that escaped. Following the Great War the sloths escaped and mutated into Mega sloths.
Isaac Garrahan is mentioned only in Fallout 76.