Did the Elves in Middle Earth know of Ents? They were supposed to be very friendly with nature and especially the forests, so they should have been aware.
In the Return of the King chapter "Many Partings", Fangorn has a brief conversation with Celeborn and Galadriel which makes it clear that they have personally met in the past (though not recently):
Then Treebeard said farewell to each of them in turn, and he bowed three times slowly and with great reverence to Celeborn and Galadriel. "It is long, long since we met by stock or by stone, A vanimar vanimálion nostari!" he said. "It is sad that we should meet only thus at the ending. For the world is changing: I feel it in the water, I feel it in the earth, and I smell it in the air. I do not think we shall meet again.
And Celeborn said: "I do not know, Eldest." But Galadriel said: "Not in Middle-earth, nor until the lands that lie under the wave are lifted up again. Then in the willow-meads of Tasarinan we may meet in the Spring. Farewell!"
Yes, they did. Gandalf had consulted with Treebeard in the past, so the existence of the Ents were definitely known among the Wise. Celeborn shows no astonishment when greeting Treebeard in the Return of the King, recognising him as "Eldest".
Historically, the Ents accredited the Elves with teaching them to speak, as Treebeard mentions the Elves "curing us of our dumbness". The Ents also took part in defeating the force of Dwarves who sacked Doriath in support of the Elves vack in the First Age.