Elmo was only mentioned in the Unfinished Tales in discussions on Galadriel and Celeborn. Because Celeborn was in The Silmarillion presented as a "kinsman of Thingol", Tolkien wished to develop this family link. One of his solutions was to establish Celeborn as the grandson of Elmo, a non-important brother of Elwë who remained behind when Elwë was lost, becoming one of the Sindar of Doriath. In this conception Elmo had a son named Galadhon, who in turn sired Celeborn and another son named Galathil, who was the father of Queen Nimloth of Doriath.
It therefore remains uncertain if Elmo would have remained in the Middle-earth legendarium.
The name Elmo has unclear origins. The first element el may mean 'star', which is found in both Sindarin and Quenya. The second element mo, although does not seem to have any direct origin in Sindarin, may have come from the words mor, meaning 'dark', mîn, meaning 'towering' or 'one', or mae, meaning 'good' or 'well'. Chronologically, these words would give the meanings "dark star", "towering star", "first star" or "good star" to his name.
If his name were to be Quenyan, the word mo could have come from mo, meaning 'one' (in reference to a person) or moru, meaning 'to hide', although it is more likely for his name to be Sindarin rather than Quenyan. These Quenyan words would give the meanings "star person" or "hiding star" to the word Elmo.
In Quenya, -mo is a frequent ending in names and titles, sometimes with significance.
- Unfinished Tales, Part Two: The Second Age, IV: "The History of Galadriel and Celeborn, and of Amroth King of Lórien"