A Protective Dog
A man and his wife have a little baby. One day, the man is caring for the baby while his wife is out shopping and he hears a noise outside. He leaves his dog to watch over the baby while he goes out to see what the problem is.
After chasing off a prowler, the man goes back inside to find the baby crib splattered with blood and the dog cowering under the crib with blood all over its muzzle. Thinking that the dog killed his baby, the man becomes enraged and kills the animal with a baseball bat.
Only then does he look on the other side of the crib. There he finds his baby lying on the floor dead, a knife in one hand and some kind of alien communication device in the other. The couple's baby had, in fact, been replaced by an agent of a hostile extraterrestrial civilization, and would have killed the man upon his return had it not been for the brave actions of his loyal dog.
Behind the Legend
This incident was reported in the 1960s by Barry Hill. He and his wife, Beth, stuck with the story until the day they died in a Georgia mental facility. A videotape was recently discovered of what was supposed to be an autopsy of the baby's corpse. During the autopsy, it is shown that the tiny creature had internal organs completely different from those of normal humans. Unfortunately, the corpse was not preserved and the videotape's veracity is in question because some experts find it unlikely that a professional coroner in the 1960s would allow a visiting teenager to sit in the background disinterestedly playing his GameBoy during as important a procedure as this.