Hi, if I put a +1/+1 on Fleecemane Lion on turn 3 with something like Oath of Ajani. Can I still pay 3WG, make the fleecemane lion monstrous when reach 5 mana? So to simplify the question, can a creature still become "monstrous" even if something else put a +1/+1 counter on it earlier?
For the purposes of monstrosity, whether a permanent is monstrous does not depend on whether or not it has +1/+1 counters on it; monstrous is merely "a designation and ... act[s merely] as a marker that the monstrosity action and other spells and abilities can identify" (C.R. 701.31a-b). For example, Fleecemane Lion does not become monstrous merely if it's given one or more +1/+1 counters (review C.R. 701.31a).
However, Fleecemane Lion's first ability can be activated whether or not Fleecemane Lion is monstrous and whether or not it has +1/+1 counters on it (that ability has no restriction on when it can be activated within the meaning of C.R. 602.1a, such as "Activate this ability only if ...", and it isn't a loyalty ability as it has no loyalty symbol in its cost [C.R. 606.2, 606.3]; see also this thread and this thread).
EDIT: Clarification after comment 5 was posted.
EDIT (Mar. 29, 2021): Some rules were renumbered in the meantime.
This is pretty much the difference between Monstrosity and Adapt. Adapt is the 'simplified' version of Monstrosity where the existence of +1/+1 counters is used to define whether or not the creature is eligible to activate that ability. But for the OG Monstrous creatures, as stated, the status of whether it is or is not monstrous is just something the players have to remember, because a creature could have counters on it and not be monstrous, and likewise a creature could be monstrous despite no longer having counters on it.
For Adapt they dodged this memory issue by tying adapt-activation-eligibility to the (non)existence of +1/+1 counters.
This is pretty much the difference between Monstrosity and Adapt. Adapt is the 'simplified' version of Monstrosity where the existence of +1/+1 counters is used to define whether or not the creature is eligible to activate that ability. [...]
For Adapt they dodged this memory issue by tying adapt-activation-eligibility to the (non)existence of +1/+1 counters.
The adapt keyword on an activated ability, just like monstrosity, is not a restriction on when that ability can be activated under C.R. 602.1a, such as "Activate this ability only if..." (C.R. 701.43a). For example, Aeromunculus's activated ability can be activated regardless of whether Aeromunculus has any +1/+1 counters on it, similarly to the fact that Fleecemane Lion's activated ability can be activated regardless of whether Fleecemane Lion is monstrous or whether it has any +1/+1 counters on it; it's just that each of those abilities won't do anything when it resolves if Aeromunculus has one or more +1/+1 counters on it or if Fleecemane Lion is monstrous, respectively (C.R. 701.43a, 701.31a). Compare both abilities with Ice Cauldron's first ability.
EDIT (Mar. 29, 2021): Some rules were renumbered in the meantime.
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However, Fleecemane Lion's first ability can be activated whether or not Fleecemane Lion is monstrous and whether or not it has +1/+1 counters on it (that ability has no restriction on when it can be activated within the meaning of C.R. 602.1a, such as "Activate this ability only if ...", and it isn't a loyalty ability as it has no loyalty symbol in its cost [C.R. 606.2, 606.3]; see also this thread and this thread).
EDIT: Clarification after comment 5 was posted.
EDIT (Mar. 29, 2021): Some rules were renumbered in the meantime.
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Want to play a UW control deck in modern, but don't have jace or snaps?
Please come visit us at the Emeria Titan control thread
For Adapt they dodged this memory issue by tying adapt-activation-eligibility to the (non)existence of +1/+1 counters.
EDIT (Mar. 29, 2021): Some rules were renumbered in the meantime.