You may exile two green cards from your hand rather than pay Allosaurus Rider’s mana cost.
Allosaurus Rider’s power and toughness are each equal to 1 plus the number of lands you control.
15/7/2006 : Paying the alternative cost doesn’t change when you can cast the spell. A creature spell you cast this way, for example, can still only be cast during your main phase while the stack is empty.
15/7/2006 : You may pay the alternative cost rather than the card’s mana cost. Any additional costs are paid as normal.
15/7/2006 : If you don’t have two cards of the right color in your hand, you can’t choose to cast the spell using the alternative cost.
15/7/2006 : You can’t exile a card from your hand to pay for itself. At the time you would pay costs, that card is on the stack, not in your hand.