Quote from WizardMN »A Judge Blog and a Cranial Insertion Article (both fairly old but should still be accurate) disagree with you:Quote from SyntheticDreamer »
You actually can't cast Zoetic Cavern from the top of your library with Garruk's Horde either for the same reason - while it's in your library, it's a land card, not a creature card, so you won't be able to cast it from there with that ability since the ability specifies that you can only cast the top card of your library if it's a creature card.
You check the legality of casting it after it has gone onto the stack (the fifth step specifically) and Horde allows you to cast a creature from the top of your library. And since the card is now face down on the stack, and it is a card, it is a creature card since that it what it is on the stack when the game checks to see if you are permitted to cast it.
The rule for casting legality has changed since then.
Now the game checks if a spell can be cast both before (601.3) and after (601.2e) it moves to the stack, and effects that allow you to cast spells with certain characteristics from zones other than hand aren't covered in 601.3 a-d.