Well, these may be syringe marks, their placement meant to emulate fangs. Such ritualistic blood-letting points towards cultists of some sort, in which case - 

Yeah, that’s probably it. Satanic cultists. Come on, Scully!

You’re not gonna tell me this is that Mexican goat sucker.

El Chupacabra? No, they’ve got four fangs, not two. And they suck goats, hence the name.

So instead this would be…?

Classic vampirism.

Bad Blood, The X Files S05E12