Coyotes and El Chupacabra
Chupacabras on mainland North America are much more likely to be described as dog-like creatures with long, sharp fangs, and a row of spines along their back, than they are to be identified as the more reptilian, bipedal beast said to stalk Puerto Rico. Pictures, videos, and even corpses of these supposed chupacabras have become relatively commonplace in Mexico and the American Southwest, and in almost every case the purportedly cryptid creature is nothing more than an unfortunate coyote suffering from mange. Mange is a skin disease caused by parasitic mites; it can cause an animal’s hair to fall out and skin to shrivel, giving it a strange, almost alien appearance. If an observer does not know what an animal with mange looks like, then it's understandable that they might mistake the afflicted creature for something monstrous.
This explanation is sufficient for many scientists who have investigated the chupacabra phenomenon. They say that a coyote with mange is likely to be debilitated, and weakened predators often choose livestock as prey, which could explain the chupacabra’s predilection for feasting on domesticated animals. This doesn't explain the chupacabra’s reported thirst for blood, nor does it explain the creature’s origin in Puerto Rico, where it is described much differently than on the mainland.
It is likely that stories of the chupacabra spread along Hispanic communities from Puerto Rico to Florida, and into the American south to the southwest and into Mexico. It is less likely that an actual cryptid followed the same migration. Adapting these stories to their own local phenomenon explains how descriptions of the creature could vary so drastically. This doesn't account for the original, unexplained sightings, of course.
So, while a misidentified mangy coyote might explain many of the chupacabra sightings in Mexico and the American Southwest, the creature that started the phenomenon remains a mystery.