Dolphmon

Dolphmon is a Champion Level Sea Animal Digimon. It's Japanese name is Rukamon.

Digimon 02
A bunch of Dolphmon helped Ikkakumon, Mega Seadramon, Cody Hida, and Submarimon protect a Destiny Stone under the sea from Black War Greymon. Black War Greymon used an attack to part the sea sending the Digimon flying.

Digimon Tamers
Something that might be Dolphmon was among the shadowed Digimon that wanted Rika Nonaka to tame one of them. A silhouette of one appeared on Henry Wong's computer.

Digimon Frontier
Despite being in the theme, the chosen kids didn't encounter one. But they are in the theme song seen with Takuya Kanbara, Neemon, and Bokomon on their backs.