Fish Cooper is a character first appearing in the spin-off show, Young Sheldon.
Fish was a small fish given to Sheldon Cooper by Meemaw to overcome his fear of animals and for Sheldon to get comfortable around pets. Sheldon initially liked Fish, due to Fish not caring about Sheldon and ignoring him most of the time, although Missy found fish boring. But once Sheldon tried to pet Fish, it bit Sheldon's finger, terrifying him. Sheldon exclaimed this to George, who then showed Sheldon that Fish was harmless by touching Fish himself. However, Fish bit down hard on George's finger, causing him to instinctively swing his finger, accidentally killing Fish.
Fish's name was given by Sheldon as a way to avoid getting attached to it.