Maester Seymour
At the beginning of combat on your turn, put a number of +1/+1 counters equal to Maester Seymour's power on another target creature you control. {3}{G}{G}: Monstrosity X, where X is the number of counters among creatures you control. (If this creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)
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Official rulings (4) · Scryfall
The value of X is determined only once, as Maester Seymour's last ability resolves.
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
If Maester Seymour is no longer on the battlefield when its first ability resolves, use its power as it last existed on the battlefield to determine how many +1/+1 counters to put on the target creature.
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.