Space fel orcs were sent into space to fight for the demonic legion. They are armed with powerful flame throwers which can target ground and air units.
They have an unspecified ability called "revenge".
It can also be seen in a bonus credits at the end of Reign of Chaos, along with zerglings, hydralisks and marines, but only if it is beaten on hard mode. At the end of the sequence, the space fel orcs are the only ones that survive the battle.