What is the mudkip joke?

Mudkip is a Pokémon introduced in the third generation of Pokémon games, Ruby, Sapphire, and Emerald. It is a Water-type Pokémon and starter Pokémon. Mudkip is known for its playful personality.

The mudkip joke is a reference to the fact that Mudkip is a Water-type Pokémon. The joke is:

What type of Pokémon is afraid of ghosts?

A Mudkip! (Mudkip scared, mudskip)

The joke is funny because it plays on the fact that "Mudkip" and "mudskip" sound similar, and because Mudkip is a Water-type Pokémon, which are typically weak against Ghost-type Pokémon.