>>26698207They're clearly a trip that symbolize renewal of life, and life coming out of death. Grimer is a lifeform created out of pollution and toxins.
Vullaby is a check, an infant, but it's wearing a skull, a symbol of death. Vultures are also carrion eaters.
Swampert resembles an axolotl, an animal famous for staying in a larval stage all life, unless being forced into transforming into its adult form. It's a symbol of rebirth and eternal youth.
See, clearly these three were designed with a common pattern in mind.
