Not that I'm aware of, although I'd guess that somewhere in her stats is the Psuedonatural template - perhaps modified a bit. She could change from a 'normal' seeming human(oid) to a scaly alien-looking one that had a tentacle or three as I recall. (One was her tongue? It's been a while since I watched Species.)
Let's see, in her altered form she could spider climb and had a tentacle attack with notable reach. Hmm, she likely had natural armor and perhaps some form of energy resistance. Maybe some fast healing? I don't recall any regen, however. Perhaps a 'disease' type touch, as the rat that at a bit from that severed thing changed into a rat variant of the template a day or two later. The change from being 'normal' to being templated (ie: able to change back and forth from the templated form) seems to require time in a coccoon as I recall, so throw that in somewhere as well - the descriptive text describing the transformation, perhaps. Like were's some can be natural while some are changed later (from consuming the flesh / blood of a templated one, perhaps, or perhaps a severe form of injury - an open wound splashed with some of the blood of the creature, perhaps - might also affect the change).
From her movements I'm guessing that Dex increased, as well as Str, Con, and Cha. Not sure if Wis increased, although I wouldn't be surprised. Int increase is highly questionable, as the creature seemed more perceptive / intuitive than tactic / strategic in its actions (after the transformation at the end).
So far we have:
2 forms, one mostly normal, one with various bonuses
- second form has 1d4 tentacles with increased range for slam attacks, trips, etc
- second form likely has natural armor increase, maybe some fast healing
- second form likely has increases to Str, Dex, and maybe Con, Wis, and Cha, probably not Int
- second form might have some form of energy resistance(s), although this might be more D&D flavor than from the movie
- second form seems to have spider climb equivalent ability
Personally, I see this and I think, "Okay, we have a Psuedonatural (Half-?)Fiendish Human," but I admit this is simplifying the situation - esp considering some of the specials of these two templates. I'd probably use these templates as a base, however, drop the spells of the Half template (if I used it rather than the fiendish template), change some of the specials (to spider climb, for instance), and remove the AL. Sil was true neutral (as any (semi-) wild creature happens to be) with strong NE (ie: utterly selfish) leanings.
Recall that she 'grew up' to the age of twelve in only 2-4 years, and then skipped puberty to go straight to early 20-ish woman in perhaps a day or two. Mentally and emotionally (excluding sexuality, which for her was more a matter of instinct than emotions) she was only a few years old. It is said that the 'conscious' that helps determine right or wrong is not fully developed until about the age of 7 - 8. Sil missed most of those years and so was arguably not developed enough (mentally or emotionally) to have a non-neutral alignment. Granted, in D&D once Int is above 2 one typically has an alignment, so for D&D purposes we would have to chose one or the other (NN or NE). Given the movie, we would have to state NE - as she killed several times without need - just because it was expedient.
Anyway, does this help any?