From the Super Mario Wiki
A Trouter (Totosu in Japanese) is a red fish that jumps straight up out of water near waterfalls in Super Mario Bros. 2. Their name is a pun on Trout, a type of fish
Super Mario series
Super Mario Bros. 2
Trouters are used many times throughout the game as platforms in which to jump across a large body of water. Stopping time with the Stop Watch makes it much easier to cross them. Although not deadly from below, they are dangerous if touched from the sides. Not only this, but if Mario, Luigi, Princess Peach, or Toad ride the Trouter back to the watery depths, they will lose a life.
BS Super Mario USA
Trouters made return in BS Super Mario USA, when they have the same role in the predecessor.
The Super Mario Bros. Super Show!
On The Super Mario Bros. Super Show!, Trouters made appearances in numerous episodes, where they were portrayed as being far more vicious than they were in Super Mario Bros. 2. Rather than merely hopping in and out of water, Trouters were shown to be fairly carnivorous with sharp teeth, and would try to eat anyone who fell into their water. A running gag throughout the show was that Trouters were usually swimming in whatever body of water that Mario and his friends either had to cross or fell into, often appearing as an unwanted surprise and scaring the group. Sometimes, to show their appetite, Trouters were depicted by wearing bibs, and holding knives and forks.
Trouters were not shown to be minions of King Koopa, as they would attempt to eat him if given the opportunity. However, it was not uncommon for King Koopa to use Trouters to his advantage, as he sometimes had them guard the waters surrounding his bases in certain episodes.
In Nintendo Comics System, Trouters appear in the stories "The Fish That Should've Gotten Away" and "Beauty and the Beach." In the comics, the Trouters featured resembled and acted like their cartoon counterparts, but were also depicted as capable of speaking.