Pygmy Suckerfish

Pygmy Suckerfish are a small, mutualistic fish which most likely lives in symbiosis with other fish.

Source
Pygmy Suckerfish are primarily caught from the schools of fish found in Northrend but have a smaller chance to be found in open water. Schools of fish, with the exception of Crystalsong Forest have about a 65% chance to drop suckerfish, while open water has about a 10% chance.

As an Ingredient
Suckerfish can be used by an alchemist to make 1 to 2 Pygmy Oil:

can be drank or used as a reagent for several other alchemy recipes:
 * Flask of Pure Mojo
 * Guru's Elixir
 * Indestructible Alchemist Stone
 * Potion of Speed
 * Potion of Wild Magic

Namesake
Suckerfish is a common name for bottom feeders, frequently found in fish tanks of enthusiasts. The name is relatively straight forward, referring to the large "sucker" on their face that serves as a mouth.

Pygmy suckerfish specifically could refer to any number of small fish, perhaps an Otocinclus such as the zebra dwarf sucker or the golden dwarf sucker. However, it is more likely a parallel to the real world Lampreys, a group of primitive, eel-like fish in which some species have sucker-like, teeth-lined jaws that they use to attach to other fish to suck blood and other bodily fluids out of them. This would explain why they are found in loot boxes with other fish.