The answer is, absurd though it may sound, A PORCUPINE.
No, I am not being funny.
Upon closer examination we find more corroborating evidence. Porcupines have a red eyeglow and in some species the eyes are circled with a reddish-coloured skin patch. The eyes of a Chupacabras are also commonly represented as black and beady. The short snout has a mammalian "wet" nose on it and a prominently cleft upperlip. Protruding out of the upper lip is a pair of gnawing rodent incisors, although the artists can show some confusion about the shape of the lower incisors. Whiskers are sometimes shown on the sides of the snout, and to top it all off, ears are sometimes shown at the back of the head as the right size and shape to be porcupine ears (Although the ears are more usually said to be not apparent to the witnesses)
It also seems the original sketch is a little misleading in that the area with quills on the back has them being shown spread too thinly, and as a result the body looks too emaciated. However, the bottom as shown is actually quite thick in proportions. Filling in the area indicated for quills with a thicker bunch of quills copletely alters the body profile.
This species not only has the red skin patch around the eye, the body has a general greenish cast to it (The quills are sometimes said to be irridescent and show as different shifting colours to the observer)
Well, first off, Natasha is still obviously built like a human female, stands like a human female, has arms and legs of the usual human proportions and used in the usual human manner, has a waist which givers curves to her figure, and she has breasts. The Chupacabras has short animal legs, no boobs, and a bottom-heavy body. Natasha Henstridge does NOT have a "Gray Alien" head or eyes, and as an alien the eyes are heavily-lidded and not obviously red or black or any other colour. She has more and diffeent kinds of hoses and attachments on her that the Chupacabras does NOT have. In short, there is VIRTUALLY NO RESEMBLANCE from an objective point of view. So if Tolentino thought the creature she saw was anything like Natashia being "Sil", she was confused or being misled by her questioner. Which is quite possible and that questioner should be fully cognizant that the process happens, since he says so himself.
And I am still not asking Benjamin Radford for $250.00-No, that would be too small a price to pay after being so misleading on the subject. I would ask for $25000.00 and that the book be withdrawn from publication. And of course that if there needs to be any court case arising from this that Mr. Radford pay court costs since he was the one that offerered the reward and initiated all this nonsense in the first place.
Best Wishes, Dale D.