A large, endangered, whale-like lifeform that lives in open space and is protected under Starfleet rules.
Gormaganders are huge creatures that drift through space much as whales move through the ocean. They are an endangered species, and Starfleet procedure calls for a ship that finds one in distress to beam it aboard and care for it.
In Magic to Make the Sanest Man Go Mad, Discovery took aboard a gormagander. Harry Mudd had hidden inside it to sneak onto the ship, where he used a time crystal to loop the same half hour over and over.