A rat is a medium-sized, long-tailed rodent. During Triad to determine if Skaara's body should belong by himself or the Goa'uld Klorel, Zipacna made the argument that Skaara's body does belong to Klorel because Goa'uld consider themselves more intelligent than humans just as humans consider themselves more intelligent than animals. To liken this, he used the analogy that a pig might consider itself more intelligent that a rat. (SG1: "Pretense")
Lt. Colonel John Sheppard believed the sound that Dr. Rodney McKay heard on Hoff were rats but they turned out to be scavengers. (SGA: "The Kindred, Part 1")