Ghost (Majora's Mask)

"Them", according to Romani, refers to the rather strange ghosts that come to invade the barn of Romani Ranch to abduct their rare, precious Romani-bred cows, which they inherited from their father. They resemble ghostly, glowing aliens, and are quite similar to Poes. According to Romani, they come at night every year when The Carnival of Time approaches. It seems aliens have an interest in rare cows with the ability to produce a fine milk and it's up to you to save the ranch from a possible invasion. If Link fails to save the cows on the first night they will abduct Romani, leaving Cremia, Romani's sister, saddened. On the second day, the ghosts will return Romani with all her memories of their existence removed, with obvious side effects on her personality. Appropriately, the Majora's Mask strategy guide calls them "alien ghosts".

Speculation has led many to suspect the Gorman Brothers are somehow involved in all this which is pretty believable considering the trouble that they are causing lately. However, as "they" are capable of wiping Romani's memories (something the Gorman Brothers don't seem to be capable of), and also taking into consideration the sheer way in which the cows are abducted, it is also likely "their" interest in the cows is a complete coincidence.

Some speculate that "They" are members of a parralell dimension. Thus, the only reasonable choice would be the Twili, quite possibly because they were a "peaceful and curious race," and it is even stated by Midna that they once left the Twilight Realm in search of the light world's interesting quantities.

Another thought is that "They" may be the Poes of Lon Lon Ranch from Hyrule, who abducted all the cows from there and needed more Lon Lon Milk. This theory is criticized for being "unreasonable" or "not thought out", but there may be a connection.

Trivia

 * It is possible to ride into the ranch partway into the invasion and still save the ranch, but this can be quite difficult, impossible without Epona's or Goron rolling speed. It is estimated that the latest time of arrival is 3am, if the ranch is still to be saved, and it is confirmed that 5am is too late.
 * "They" are likely inspired by the Kelly-Hopkinsville encounter of 1955, a well-known and well-documented alleged Close Encounter event in the history of UFO incidents. One testimony describes the beings from this encounter as, "a luminous, three-and-a-half-foot-tall being with an oversized head, big, floppy, pointed ears, glowing eyes, and hands with talons at their ends. The figure, either made of or simply dressed in silvery metal, had its hands raised." The creatures were also said to have moved by hovering, due to having legs that appeared to be atrophied. This generally constitutes with the appearance of "Them". Also, both incidents occur in a rural farm area.
 * They may also be based on the flatwoods monster due to there similar appearance to the monster and how both Aliens' spaceships are flying balls of light.