In that case, I don't know. There's nothing in the ruleset I can find to indicate anything could give you Chupacabras besides missions and Land Surveys.
I suppose we'll have to wait for Solarius to say what it might be, unless you can add more details than just 'an event that gave me a Chupacabra'. Like, did you have to process something in the workshop, did the critter arrive 'in the mail', anything?
Is the 'current one' 2.5, or are you perhaps using the Github version, so partial 2.6? Are you using any other mods? I took a brief look at Arsenal Additions, and that one doesn't seem to have free Chupacabras, either.