Lintips

Lintips are a mysterious race that appears in the Tom Bombadil related poem; Once Upon A Time.

Background
They are apparently a very small race and smell mousy. Back in the Elder Days, the lintips traversed the grass-forests gathering dew. Tom Bombadil encounthered them when they came to collect dew too early. Tom had stop to listen, and warned them to enjoy the dew, but mind his feet. The lintips laughed than ran away. This saddened Tom who wanted them to stay. They were the only creatures that wouldn't stay to talk to him, and he wondered what they hid. They were apparently shy or secretive and wouldn't divulge what they do and what they are.

Behind the scenes
John Rateliff notes;

"...the lesser spirits that accompanied the Valar when they descended into what was later called Arda, the created world...These lesser spirits are de-emphasized in the later forms of the mythology, but they never entirely faded away; as late as 1965 JRRT published a stray Bombadil poem (the third featuring that character, by far the best, and the only one omitted from THE ADVENTURES OF TOM BOMBADIL), 'Once Upon A Time', which includes the lintips, elusive little creatures even Bombadil doesn't know much about (cf. the far more menacing mewlips of another, much earlier poem). In the end, all these minor Maiar-like folk seem to have been subsumed into the general category 'fay' (which in the early stages of the legendarium is distinct from fairy, the latter then being a synonym for elf)."