Ylmir, at least in Tolkien's earlier works, was the Gnomish (Noldorin) translation of Ulmo.
 See also
J.R.R. Tolkien, Christopher Tolkien (ed.), The Lays of Beleriand, "I. The Lay of the Children of Húrin: III. Failivrin", p. 93
J.R.R. Tolkien, Christopher Tolkien (ed.), The Shaping of Middle-earth, "II. The Earliest 'Silmarillion' (The 'Sketch of the Mythology')", p. 13