Hmm... something along the lines of a failed experiment from the C'Tan, or possibly a warrior breed made by the Slomon K'Hara might work. If you want to do something with a C'Tan slave species, then I suggest talking to Virus- for a Slomon K'Hara warrior creation, talk to Treewyrm. If the idea is good enough, we might be able to write it into Nomad lore.
EDIT: Did I say failed experiment? Judging from how the Phantoms usually act, the C'Tan might be perfectly happy with a vicious bloodhound type thing, possibly modifying controls for them so they could operate a ship. Of course, you couldn't dock on the Temple of the Revenent, that would be far too dangerous- the beast likely don't discriminate and attack everything. Consider using Planet Gammu and New Haven Station as places to dock or spawn from, I can't see anything else taking you.
Honestly, since we're looking at this from a sci-fi viewer stand point, its not scary. But in real life, if you saw ANY alien, you'd be scared jassi-less. Unas, Space Vampire *cackle* (Sorry, Linkus), or weird Predator looking things. I say, just come up with some weird alien from Gaia that jacked a ship carrying researchers or rich people from the liner above and then flew off. Just make a name up, since there isn't an "alien directory" its not like people could argue with something from Gaia.