Intelligence from another planet with advanced technology
In the thirties, Mars and the Moon were the most popular candidates for hosting aliens but it seems that we have to go beyond the solar system to find the first planet with similar living conditions than the Earth. Though scientists last century thought the planet Mars might be a good candidate, and some even thought they detected a huge canal system stretching across the planet, recent probes sent to Mars have failed to detect even bacteria-like creatures, let alone a civilization capable of producing a flying saucer. With the rest of the planets in our solar system seemingly too hot or cold, the best hope for intelligent life seems to be across the void of interstellar space in other parts of our galaxy. If we do assume there is intelligent life in places other than Earth, where might they be? In an attempt to detect intelligent life beyond our solar system researchers have conducted a number of SETI programs trying to use radio waves to detect the existence of other civilizations. So far no SETI program has been successful in finding intelligent life, but there are millions of stars in our galaxy alone that might have planets that could harbor life and carefully looking at each one of them will take a long time.
Time-travelers
The chupacabra, or aliens that we are seeing would be humans of the future.... our own children.... travelling through time in order to prevent the chemical crisis that caused them to evolve into "chupacabras". This would explain the "time-line" of sightings and the secrecy of the U.S. government regarding their findings. Our future-humans arrive at times when chemical progress is at a peak - in order to either warn us or attempt to prevent the progress of the technology that will go awry and cause physical mutations. What might cause such a leap in evolution, to create the unusual appearance of chupacabra? Is it a large-scale chemical war? Instant, single-generation mutation? Or a slow, gradual "morphing" of physical structure in order to adapt to our habitat - which we created through ignorance? Is it a positive change, or a negative change? Certainly, chupacabra is not attractive to us, pre-historic man.
Psychic phenomenon
Aliens are only the results of hallucinations caused by numerous alterations of the mind of witnesses. This argument is used in most cases of paranormal apparitions wether it is a ghost, a Bigfoot or an alien.
Hidden earthly intelligence
According to this theory, what we call aliens would be pre-flood civilizations that have found a refuge in the depths of the sea or underground.
