Zephyr Fish

It comes out of ordinary fishing in any water at any depth, so no biome setup or special pool improves your odds beyond raising fishing power. Realistically it turns up as a surprise during a long run of quest fishing rather than something you chase. If you do want it deliberately, the answer is a better rod, better bait and more casts, not a clever location.

It drifts along behind you and does nothing else at all. Modern versions teleport it to you when you outpace it, which is the only reason it is usable; on the older handheld and console builds it never catches up, so it gets stranded in whatever cave you left it in and you spend the trip resummoning it.

Zephyr is the Greek west wind, personified as the god Zephyrus, and the word has been borrowed since for ships, trains and aircraft. The pet sprite itself is a recolor of the Flying Fish enemy.