In Silmarillion drafts from the 1930s, the son of Dan and leader of those Danians who entered Beleriand (LR/119, 263). In The Etymologies, Tolkien did not explain this name other than to say it derived from the same root ᴹ√DAN as his father’s name (Ety/DAN). Elsewhere, Tolkien said the primitive form of this name was ᴹ✶Ndanithārō “Saviour of the Dani”, and gave derivatives of this name in every language but Danian (LR/188).
The Danian form most closely resembles Ilk. Denithor, and may have had a similar development. In particular, the long [[dan|[ā] likely became [ǭ]]], perhaps shortening later when it became the vowel of the final syllable. The [a] in the first syllable was probably [[dan|mutated to [e] by the following [i]]], though it is unclear why the [i] would then become [e]. Finally, the initial [[dan|[nd] would simplify to [d]]].
Note that, according to the Comparative Tables (PE19/23), the medial [θ] (“th”) in this name [[dan|should have voiced to [ð] (“dh”)]]. Perhaps this name was archaic in this one aspect, or perhaps the name was partly Ilkorinized.