What’s special about Primal Clay is that you have a choice as to what it hits the board as. For 4 mana, you get to choose if it enters the battlefield as a 1/6 Wall, a 3/3 vanilla creature, or a 2/2 flying creature. Once it’s on the board, it remains that for as long as it remains there.
Years ago, this was actually a pretty fascinating card, and it was a playable card in Limited for a long time, mostly because it could become a cheap 1/6 Wall or a 2/2 Flyer. Even in Magic 2013 limited this was a playable card, and no longer a rare. It’s a fun little card from Magic’s past that’s stuck around a long time. While it certainly has not been Constructed playable for a long time, it’s still a fun card to look at.