Deviate Fish

The Deviate Fish is prized for its mutagenic properties. It can be eaten raw, cooked into Savory Deviate Delight, or used to make Elixir of Giant Growth potions by an alchemist.

Deviate fish are small fish living in the streams and waters near the Wailing Caverns. They are tiny because they have been altered by the deviate magic of the Wailing Caverns. They are a full-grown, but remarkably little, version of a common freshwater species of trout. The deviate fish are about 2 inches long, whereas a normal adult trout grows upward of 1 foot long. Furthermore, deviate fish radiate faint auras of transformation magic. Consuming one of these fish, either alive or cooked, produces various effects, usually "reduce person" which lasts for two minutes. They are very common throughout the Shrine of the Scarabs section of the caverns (WoW RPG, not in the online game). They do not survive more than a day or so outside the Wailing Caverns, and once dead their power fades after several hours.

Source
Deviate Fish are caught with the fishing skill. They are found in five distinct bodies of water in the Barrens: Catches from these waters are approximately 30% Deviate Fish.
 * in the three oases,
 * the Forgotten Pools northwest of the Crossroads,
 * the Lushwater Oasis near the Wailing Caverns,
 * and at the Stagnant Oasis near Ratchet,
 * and in the Wailing Caverns, both
 * within the instance and
 * in the waters in the entryway.

Schools of Deviate Fish spawn in the three oasis. Catches from the schools of Deviate Fish are approximately 90% Deviate Fish. There appear to be four schools of Deviate Fish spawned concurrently among the three oases, in any of four spots in each oasis, for a total of twelve possible locations. When one is fished out, another immediately spawns at another location - any empty spot, other than the one just fished, in any of the three oasis. Since four spots, including the one you just fished out are considered to be occupied, each empty spot has a one in eight chance to spawn the next school. If no one is fishing the other oases, you will shortly empty the schools in your oasis and have to travel to another to find a new school.

The waters within the instance may be preferred because mobs can be cleared, and hostile players from the other faction cannot enter your instance.

This item can also be commonly found inside the rewarded from completing Marcia Chase's daily fishing quests in Dalaran. You may get 2-4 Deviate Fish from the bag.

Effects
Eating a (raw) deviate fish will have a random effect on your character. Possible effects include:
 * Healthy Spirit - increases your character's spirit by 5 points for 30 minutes.
 * Invigorate - instantly heals your character 383 to 517.
 * Party Time! - a magic debuff that causes the character to perform random emotes for 2 minutes. Your character will /clap, /cheer, /chicken, /lol, or /dance every 10–15 seconds, possibly interrupting spells or other actions.
 * Rejuvenation - rank 4 of the druid buff by the same name, heals your character by 45 points every 3 seconds for 12 seconds.
 * Shrink - a curse debuff that reduces your character's apparent size, as well as their strength and stamina, for 2 minutes.
 * Sleepy - a magic debuff that sleeps your character for 30 seconds.

Alchemy

 * Elixir of Giant Growth

Cooking

 * Savory Deviate Delight