A Grettin is a crossbreed (usually made using the gene splicer) between a Grendel and an Ettin. As Ettins do not naturally exist in Creatures 1, Grettins are therefore usually confined to Creatures 2 and Creatures 3 - in Docking Station, they can come through the Warp from time to time but without C3 being installed they will either appear invisible (if you have no third-party Grendel or Ettin sprites installed) or will default to whatever is available.
The term 'Grettin' is also used to refer to both species at once, as in the Grettin Switch Agent available for Creatures 3, which allows users to turn the Grendel and Ettin mother on and off.