Seadramon

Seadramon was the fourth digimon that the DigiDestined encountered (in order Kuwagamon, Shellmon, and 2 Monochromon). He's a giant sea serpent with greenish-blue skin, a red tail, and a yellow head.

Digimon Adventures 01
During their first night in the Digital World, the DigiDestined come across an abandoned trolley car on the shores of Dragon's Eye Lake, where they decide to spend the night. However, Tai and Agumon accidentally strike a large, red, leaf-like object with an ember from the fire, and it turns out to be the tail of a very angry Seadramon, who retaliates by dragging the chunk of shore with the trolley car on it to the middle of the lake. Matt is the only one not on that part of the shore and leaps into the water to save the others. Seadramon notices him and grabs him with his coils. To save him, Gabumon digivolves to Garurumon for the first time and battles Seadramon, seeing him off. It is implied by its Digimon Analyzer profile that the MegaSeadramon in Myotismon's army is this same Seadramon.

Trivia
Seadramon's rookie form is called Betamon. His Ultimate form is called MegaSeadramon. Finally, his Mega form is MetalSeadramon.