Marill, the Water Mouse Pokémon. Its sensitive ears can detect distant sounds and the round part of its tail can expand and contract, assisting Marill in the water.
– Ash’s Pokedex
When its design was first shown, apparently Marill looked like a blue Pikachu, that it was referred to as “Pikablu”, and mistakenly thought as related to Pikachu.
In the anime, Ash’s temporary companion Tracey had a Marill. In HeartGold/SoulSilver, your friend character always has a Marill following him/her. So if you picked the male character, the female player character, Lyra, who would be your childhood friend, would have a Marill; if you picked the female character, the male player character, Ethan, would have a Marill.
Stat-wise, it has a lot of HP, especially for a not fully evolved Pokemon. Its defenses are decent as well, but at a cost of having pretty low Attack and Sp Attack, and poor Speed. Marill can have Sap Sipper in the Dreamworld, making it immune to Grass, one of its weaknesses, leaving it only weak to Electric.