Undersized Chihuahuas - there is no official name for them other than that. People who want to charge you an arm and a leg for a dog considered "faulted" by it's AKC standard will call them Teacups - but it's just a pretty name for something that a reputable breeder would look at as something only to be sold as companion quality and most would mark their price DOWN not up.
Mine is 2.5 pounds. She still has some growing to do. She will probably be normal sized once she is a bit older. Shes still only 10 months! She wasn't bred to be tiny, was never sold as a pocket pup, and her parents/bloodlines are 5-6 pounds.