Whenever you cast a spell, you may put it on the bottom of its owner's library. If you do, reveal cards from the top of your library until you reveal a nonland card. You may cast that card without paying its mana cost. Then put all revealed cards not cast this way on the bottom of your library in a random order. This ability triggers only once each turn.
Neera, Wild Mage has received a minor update to its Oracle text to clarify its function. Specifically, if you choose not to cast the nonland card, it will be put on the bottom of your library along with the other revealed cards.
10 Jun 2022
You must have legal targets to cast a spell with targets, even if you intend to put it onto the bottom of your library this way.