Goldeen in depth
The Tosa horn fish
The original Japanese name, トサキント, comes from tosakin, a variety of goldfish developed in Japan from ryukin and famous for an enormous tail that spreads out like a fan and means the fish has to be viewed from above, not from the side, because its profile does not show it off. The breed takes its name from Tosa, the old province that is now Kōchi, and was declared a natural monument in 1969. That is where the fins described by the Pokédex come from: they are not made up, but the feature this fish is bred for.
The English name Goldeen combines goldfish with queen, the queen, following the line of aquatic nicknames attached to it by the Pokédex entries. French takes a different route with Poissirène, from poisson and sirène.
What it was in Generation I
In Red and Blue, Waterfall was exclusive to it and its evolution. That did not last long: from Generation II onward, it became an HM and half the Pokédex learned it, taking away the only thing that set this line apart. The same fate that befell Magikarp with Splash, only backwards—Goldeen had something useful taken away.
It has had visible gender differences since Generation IV: females have a shorter horn than males. It is one of the first cases used to introduce the system, alongside species whose differences also appear in an appendage on the head.
What to do with one
If it appears early in the playthrough, its argument is one of timing. Horn Attack at 15 arrives before most decent physical moves at that point in the game, and Supersonic at 5 causes enough confusion to steal turns on the early Water routes. After that, the curve flattens out: between 20 and 30 it only adds Agility, Aqua Ring and Flail, and you have to wait for Waterfall at 35 to get something that hits again.
Compared with Remoraid, the other small freshwater fish that evolves midway through the game, Goldeen goes physical while Remoraid goes special, and neither has the stats to justify using them once they evolve. The real difference is that Remoraid changes type upon evolving and Goldeen changes nothing.
Borrowed Megahorn
The strongest move in its repertoire, Megahorn, was not originally its move: it began as the signature move of Heracross in Generation II. A goldfish ending up able to learn it only makes sense because of the horn, and the result is strange—a pure Water-type whose best attack is Bug-type, with 120 power and 85 accuracy, available at level 45 on a Pokémon with 67 Attack that cannot get everything out of it. Against Psychic- and Dark-types, it is the only thing it has, and it works better than its stat sheet would suggest.
The detail that completes the portrait appears in the last Pokédex entry: some Trainers, drawn in by the size of the horn and the shape of the fins, train only Goldeen. It does not say they win.