Wailord in depth
From hatchling to king
The pair of Japanese names tells the whole line by changing one syllable. ホエルコ is hoeru, from the English whale, plus ko, hatchling; ホエルオー replaces that ending with 王, king. English plays a different trick: Wailord contains whale, but written as wail, and then adds lord on top. Spanish translates nothing and keeps the English form, as it does with almost the entire third generation.
The model is the blue whale, and the design gives up on details: the two blowholes on top of its head, the throat pleats of a rorqual, one small eye, and that's it. The striking part is the scale. At 14.5 metres and 398 kilos, its resulting density is that of a float, not an animal, which fits the entry describing it jumping and crashing down onto the water to knock out anything underneath. A real body with that mass wouldn't bounce.
The key to the Sealed Chamber
Wailord's most memorable role in Hoenn has nothing to do with battling. In Ruby, Sapphire, and Emerald, you have to have it in your party alongside a Relicanth to open the Sealed Chamber, which is the step before the three Regis. Two specific species, chosen because one is the largest and the other one of the oldest in the game, and without them the optional content stays locked. It's one of the few times the series asks you to catch a particular Pokémon to make something else happen.
Then there's the donkey work. It learns Dive by level-up, at 30, and can use Surf, Strength, and Waterfall from TMs, so a team that wants to move around Hoenn's seas usually ends up dragging one along. Taking up a slot hurts less when that slot was going to be spent on field moves anyway.
Water Spout already had an owner
The move that defines the species isn't exclusive to it. Kyogre also learns it, at level 90, and fires it with 150 Special Attack and Drizzle activating the rain as soon as it hits the field. Wailord reaches the same move with 90 Special Attack and no ability to multiply it. They're both whales from the same generation, and the game made it clear which one was meant to be taken seriously.
It does have one corner of its own, and it's on the special side. Amnesia, at level 44, raises Special Defense by two stages at once, and Rest, at 39, heals it completely; with that 170 HP behind it, a Wailord that manages to chain the two can take special attacks much better than its 45 base suggests. There's no fix for physical hits. Earthquake, Body Slam, and anything with reasonable Attack cut through those same 45 without anything in its movepool that raises Defense.
One last detail from its starting moveset: at level 1 it knows Splash, the move that does absolutely nothing, and Soak, which turns the opponent into a Water type. The biggest whale in the game starts its career with two useless moves and a Growl.