Wailord is a species of Pokémon in the series of the same name. They first appeared in the Generation III video games, where you (being the hero), could add them to your Pokédex after you evolve Wailmer. They are noted as being the largest (in size) Pokémon in the series, being as high as 47 feet and 7 inches. However, they are oddly only the sixth heaviest Pokémon despite being the largest. It has been speculated that the cause of this is because the species resembles a blimp, which are light in terms of weight.
While finding a Wailmer is usually common, it is rare to find a Wailord in the wild, as they only appear in one or two places depending on which generation it is. In the Generation III video games, you'll be able to find one on route 129 only by surfing, and in the Generation III video games, you'll be able to find one on route 223 and 230 only by fishing with the super rod.
Wailords look similar to Blue Whales or other types of whales, and as stated above they are also similar to blimps, which is probably why they are so light while being so large. The word Wailord is a mixture of the words whale and lord.