I don't remember if this was a case of Unsolved Mysteries or some other paranormal show but I think it was either about vampires or werewolves. The beginning part of the dream had two people planning to meet someone in a dark alleyway. Then suddenly a vicious, hissing monster pops out of no where and attacks one of the people and one escapes. And then you saw police finding blood drained bodies everywhere

I don't remember if this was a case of Unsolved Mysteries or some other paranormal show but I think it was either about vampires or werewolves. The beginning part of the dream had two people planning to meet someone in a dark alleyway. Then suddenly a vicious, hissing monster pops out of no where and attacks one of the people and one escapes. And then you saw police finding blood drained bodies everywhere

It must have been on another paranormal show, since it doesn't sound familiar to me. Evidently, no such story was ever on UM.

The only Unsolved Mysteries episode I recall with drained bodies was the Chupacabra one, when it would drain the blood and eat the organs of farm animals.

I don't remember if this was a case of Unsolved Mysteries or some other paranormal show but I think it was either about vampires or werewolves. The beginning part of the dream had two people planning to meet someone in a dark alleyway. Then suddenly a vicious, hissing monster pops out of no where and attacks one of the people and one escapes. And then you saw police finding blood drained bodies everywhere

Are you mis-remembering the disappearance of Susan Walsh?

http://unsolvedmysteries.wikia.com/wiki/Susan_Walsh