From Bulbapedia, the community-driven Pokémon encyclopedia.
Corphish (Japanese: ヘイガニ Heigani) was the third Pokémon caught by Ash during his travels through Hoenn, after Taillow and Treecko. Its voice actor in Japanese is 小西克幸 Katsuyuki Konishi, and its English voice actor is Madeline Blaustein.
Ash caught Corphish in the episode Gone Corphishin'!. Ash and his friends were camping on a beach, and Corphish was attacking them from underneath the sand. Eventually its identity was revealed and Ash battled it, catching it.
Corphish has a rather quirky personality. It tends to get jealous of Ash's other Pokémon, and it enjoys being the center of attention. In Go Go Ludicolo!, it was worried that Ash's newly evolved Grovyle would get more attention from Ash because of its higher level. Like its trainer, it is headstrong and stubborn. In the episode Candid Camerupt!, Max borrowed Corphish to use against Vivi Winstrate's Marill; it jumped on Marill with a powerful attack, instantly defeating it, to Max's embarrassment.
For more information on this Pokémon's species, see Corphish.