The chupacabra is a creature believed by some to inhabit many parts of the Americas. The name "chupacabra" means "goat sucker" in Spanish. This descriptive name was given to the creature due to the claim that it was responsible for the deaths of goats by exsanguination (removal of the blood).
The creature is usually described as the size of a large dog with scales and a row of spines on its back. It moves by hopping like a kangaroo or rabbit. Interestingly the creature is often described as bipedal (walking on two feet). The chupacabra has been reported in Puerto Rico, Chile, Argentina, Bolivia, Dominican Republic, Columbia, Honduras, El Salvador, Panama, Brazil, Nicaragua, Peru, Mexico and even as far north as the southern United States.
This About.com link details a history of reported sightings going back to 1996.
Whilst the chupacabra is widely accepted to be an actual creature due to an abundance of circumstantial evidence. The evidence mostly consists of the remains of livestock, especially goats being attributed to an attack by a chupacabra. Less abundant are eyewitness accounts of the creature. Photographic evidence is scarce and highly dubious. Physical remains are non existent.
It is reported that the chupacabra craze began in Puerto Rico after a series of farm animal killings in 1996. The animals were reportedly found with puncture holes in their chests and drained of blood. The draining of blood was never confirmed by an autopsy, but rather was the opinion of a veterinarian who investigated the early cases.
Eyewitness accounts increased dramatically after the publicity following the 1996 Puerto Rico cases. This kind of increase in sightings following publicity is common with all kinds of unusual creatures and paranormal phenomena. It doesn't lend much support to the argument, but rather adds to the confusion due to the diversity of the reports.
A good investigation of the topic has been undertaken by Benjamin Radford and reported on bbc.com. Whilst his findings could be considered inconclusive, he believes that the original chupacabra stories were influenced by science fiction.
Another investigation is outlined in the Youtube video "Chupacabra Mystery Finally Revealed".
Due to the lack of physical evidence and the ambiguity of eyewitness accounts, the chupacabra is most likely a nice easy "explanation" for a whole series of things ranging from cases of mistaken identity of common animals to blatant deceit in pursuit of attention.