The Two Towers, “Treebeard”. It is an old list of a living things that ents learned when they were young. Treebeard recited this when he met Merry and Pippin, trying to figure out where hobbits were in the list. When he bid goodbye to them at the fall of Isengard, he had added them in.

Learn now the lore of Living Creatures!
First name the four, the free peoples:
Elders of all, the elf-children;
Dwarf the delver, dark are his houses;
Ents the earthborn, old as mountains;
Man the mortal, master of horses:

Beaver the builder, buck the leaper,
Bear bee-hunter, boar the fighter;
Hound is hungry, hare is fearful…

Eagle in eyrie, ox in pasture,
Hart horn-crowned; hawk is swiftest,
Swan the whitest, serpent coldest…

Ents the earthborn, old as mountains,
The wide-walkers, water drinking;
And hungry as hunters, the Hobbit children,
The laughing-folk, the little people.

Provided by: NenyaGold

A shortened version of the poem is used in The Two Towers in the chapter of The Voice of Saruman:

Ents the earthborn, old as mountains,
the wide-walkers, water drinking;
and hungry as hunters, the Hobbit children,
the laughing-folk, the little people,

Provided by: legolasfanlotr