形見持ちのゴルゴン

接死
{5}{B}{B}：怪物化１を行う。（このクリーチャーが怪物的でないなら、これの上に＋１/＋１カウンターを１個置く。これは怪物的になる。）
形見持ちのゴルゴンが怪物的になったとき、対戦相手がコントロールするゴルゴンでないクリーチャー１体を対象とし、それを破壊する。

念赠蛇发妖

死触
{5}{B}{B}：蛮化1。（如果此生物未蛮化，则在其上放置一个+1/+1指示物且它已蛮化。）
当念赠蛇发妖蛮化时，消灭目标由对手操控的非蛇发妖生物。

Keepsake Gorgon

Deathtouch
{5}{B}{B}: Monstrosity 1. (If this creature isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
When this creature becomes monstrous, destroy target non-Gorgon creature an opponent controls.

Pioneer Masters

Conspiracy: Take the Crown

Deathtouch
{5}{B}{B}: Monstrosity 1. (If this creature isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
When Keepsake Gorgon becomes monstrous, destroy target non-Gorgon creature an opponent controls.

Theros

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.

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.

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.