EBE stands for Extra-terrestrial Biological Entity. The word was probably first used in 1949 when a surviving crew member of a crashed UFO was found near Roswell, New Mexico. (A minority of sources claim that a first survivor was already found after a crash in 1947).
When more crash survivors were found at subsequent crash sites, and when the US Government started exchanging 'ambassadors' with alien civilisations, the designations were changed to EBE-1, EBE-2, etc.

The 1954 SOM1-01 document, e.g., refers to two types of EBE:

a. EBE Type I. These entities are humanoid and might be mistaken for human beings from the Oriental race if seem from a distance. They are bi-pedal, 5-5 feet 4 inches in height and weigh 80-100 pounds. Proportionally they are similar to humans, although the cranium is somewhat larger and more rounded. The skin is a pale, chalky-yellow in color, thick and slightly pebbled in appearance. The eyes are small, wide-set, almond-shaped with brownish-black irises with very large pupils. The whites of the eyes are not like that of humans, but have a pale gray cast. The ears are small and set low on the skull. The nose is thin and long, and the mouth is wider than in humans, and nearly lipless. There is no facial hair and very little body hair, that being very fine and confined to the underarm and the groin area. The body is thin and without apparent body fat, but the muscles are well developed. The hands are small, with four long digits but no opposable thumb. The outside digit is jointed in a manner as to be nearly opposable and there is no webbing between the fingers as in humans. The legs are slightly but noticeably bowed, and the feet are somewhat splayed and proportionally large.

b. EBE Type II. These entities are humanoid but differ from Type I in many respects. They are bi-pedal, 3 feet 5 inches - 4 feet 2 inches in height and weigh 25-50 pounds. Proportionally, the head is much larger than humans or Type I EBEs, the cranium being much larger and elongated. The eyes are very large, slanted, and nearly wrap around the side of the skull. They are black with no whites showing. There is no noticeable brow ridge, and the skull had a slight peak that runs over the crown. The nose consists of two small slits which sit high above the slit-like mouth. There are no external ears. The skin is a pale bluish-gray color, being somewhat darker on the back of the creature, and is very smooth and fine-celled. There is no hair on either the face or the body, and these creatures do not appear to be mammalian. The arms are long in proportion to the legs, and the hands have three long, tapering fingers, and a thumb which is nearly as long as the fingers. The second finger is thicker than the others, but not as long as the index finger. The feet are small and narrow, and four toes are joined together with a membrane.

It is not definitely known where either type of creatures originates, but it seems certain that they did not evolve on earth. It is further evident, although not certain, that they may have originated on two different planets.

After a while, these types of designations were abandoned.