Or an otter with fins.

I decided to go check if that's true and it turns out that there's actually no Pokédex entry which says that.

The Silver/Soulsilver entry says: For no reason, it jumps and splashes about, making it easy for predators like Pidgeotto to catch it mid-jump.
The Moon entry says: Its reckless leaps make it easy pickings for predators. On the bright side, many Pokémon enjoy longer life spans, thanks to Magikarp.