"Mascots" aren't usually just a creator's personal favorite, but something selected for its broader appeal and how well it represents and summarizes a setting. Chainsaw Kid is technically the first monster I came up with, has proven to be the most popular, is easy to recognize and is a very typical Mortasheen creature. A giant mutant flea is a simpler concept that appeals more to just people like myself who obsess over bugs and parasites; it wouldn't really be a good example of Mortasheen as a whole.