In the "Fog on the Barrow Downs" chapter of The Lord of the Rings, Tom Bombadil retrieves Merry's five ponies, which had scattered whilst the hobbits were in the barrow. He calls them each by name.
Merry...had not, in fact, given them any such names, but they answered to the new names Tom had given them for the rest of their lives.
The narrator uses the word "given" there, but the narration is from the hobbits' perspective, so it may be that Bombadil simply appeared to have given them those names. Did he "assign" names to the ponies, or did he simply know which names they preferred?