elpablo wrote:Correct me if I'm wrong, but I believe it actually adds an additional "type" to the monster so for fleecemane it would actually be a Monstrous Creature - Cat, in the type area of the card.
Negative, no modification to the creature type.
The official rules for monstrosity are as follows:
701.28. Monstrosity
701.28a "Monstrosity N" means "If this permanent isn't monstrous, put N +1/+1 counters on it and it becomes monstrous." Monstrous is a condition of that permanent that can be referred to by other abilities.
701.28b If a permanent's ability instructs a player to "monstrosity X," other abilities of that permanent may also refer to X. The value of X in those abilities is equal to the value of X as that permanent became monstrous.
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
(Note this was copied and pasted from the rules addendum that was sent out prior to Theros pre-releases.)