A Jock-Xenomorph is a Space Jockey (Mala'kak) that has been infected by a Xenomorph Facehugger. It died hundreds of years before the crew of the USCSS Nostromo arrived at the Space Jockey's space ship. It is now considered most likely an adult version of the Proto-Xenomorph, or the adult of a less primitive descendant of said variant.
There is no report of another one.
Interestingly enough when it emerged, it seemed to have reached its adult stage at a greater rate than usual. Also of note, the existing Xenomorphs of the local hive seemed to be hostile towards the Jock-Xenomorph due to the fact that the Xenomorphs are based on a eusocial society, meaning any change in scent will cause said being (in this case the Jock-Xenomorph) to be unwelcome in the hive. In a similar manner to when a Xenomorph Drone transforms into a Praetorian Guard, the said Xenomorph Warrior produces a scent which is unknown to the hive and the other Xenomorphs of the hive attack said Warrior.