Stuart was an insect-like animal kept as a pet by Harcourt Fenton Mudd.
During his captivity by the Klingons in 2256, Mudd trained Stuart to steal food for him and spy on his fellow prisoners via a listening device attached to Stuart's body.
In December of that year, Mudd used Stuart to spy on Captain Gabriel Lorca and Lieutenant Ash Tyler. After discovering this, Lorca destroyed the listening device and threw Stuart against a wall, nearly killing him. Lorca and Tyler later escaped, leaving Stuart and Mudd behind. (DIS: "Choose Your Pain")
Mudd later used Stuart in a devious scheme to free himself from Klingon captivity. However, afterwards Stuart followed his natural instincts and escaped into the wild. (DIS: "Magic to Make the Sanest Man Go Mad")