Mohmoo

Mohmoo is a giant "sea cow" (literally) that was brought from the Grand Line by Arlong's crew. It was he that destroyed Gosa Village. However when he encounters Luffy, Sanji and Yosaku's boat, he is promptly beaten, and used to tow their boat to the island. Later, during the battle at Arlong Park, he called upon by Hatchan (his caregiver) to the fight the Straw Hats. Despite memories of his first defeat, he does as told, only to be used in the Gomu Gomu no Kazaguruma, an attack by Luffy that takes out most of Arlong's forces, and then was thrown out of Arlong Park in the process, never to be seen again. Momoo is possibly an ophitauros.
It can make a "mooo"ing sound like any cow. Apparently it got no problem staying on land since Arlongs crew pulled it right through the Gosa Village without it getting any known damages.

First Appearance: Chapter 73; Episode 31