Poison

"The poison type (Japanese: どくタイプ Doku type) is one of the seventeen elemental types. Notable Trainers that specialize in Poison-type Pokémon include Koga, Aya and Janine of Fuchsia City, It should be noted that all of these trainers are related. Some villainous teams, such as Team Rocket, also frequently use Poison-type Pokémon." - Bulbapedia

Also head_starly's primary defense. He has ingested million upon millions of gallons of it. He is also insane as hell. His "poison" is just tree sap. Don't tell him or he might meltdown. Best to just play along.