I heard that on certain copies of the game there is an extra boss battle. How to activate it, I got no clue. It's just a rumor anyway.
EDIT: I searched it and here it is. This is also how you get to the secret garden.
Secret garden
Successfully complete all four modes and have a large amount of stamina. Then, climb the back of the temple you first started in. Climb up the ivy and follow the trail all the way up and around the ledge to the right. Jump across at the other end of the ledge to reach the next set of ivy. Continue on until you get to another ledge above that you must climb around. Once at the other side of this ledge, climb to the right then shimmy across another ledge that leads behind a column. Jump to the ivy-covered column, then continue climbing. The ivy will lead to a walkway. Climb the stairs and walkway to the left of the garden. A sequence will begin and the seventeenth Colossi will appear. Defeat the Colossi to open the path to the top of the temple. Note: Eating fruit from the trees in the garden causes damage.
It is possible to reach the Secret Garden early. Find the patch of ivy behind the Temple you start at (almost directly under the huge bridge you came in on). If your grip meter is high enough, you can just jump straight up the side until you reach the first ledge, which has a small patch of leaves on it. Position yourself directly under that patch of leaves. Continue to hold Grip/Duck and jump straight up. Your character will immediately duck in those leaves, allowing you to refill any lost grip. Then, continue around and up the side until you reach the path leading to the garden.