Grunthors are large dinosaurs that you will come across while exploring Sargasso. Killing 5 of them is necessary to unlock the "Extinction Event" trophy. This is the missable trophy, so it's best not to try to skip the Grunthors, just kill any of them that get in your way. The two Grunthors will not be available until subsequent visits to the planet.
Note: There is a way to quickly get this trophy on your first visit to a planet (maybe it will be "fixed" in one of the updates). You can kill the same Grunthor five times during your first visit to the planet. When you kill a Grunthor, simply jump into the swamp. Both you and Grunthor will respawn, and you will be able to kill him once again.
