Vliesmähnen-Löwe

{3}{G}{W}: Monstrum 1. (Falls diese Kreatur nicht monströs ist, lege eine +1/+1-Marke auf sie und sie wird monströs.)
Solange der Vliesmähnen-Löwe monströs ist, hat er Fluchsicherheit und Unzerstörbarkeit.

León melena de lana

{3}{G}{W}: Monstruosidad 1. (Si esta criatura no es monstruosa, pon un contador +1/+1 sobre ella y se convierte en monstruosa.)
Mientras el León melena de lana sea monstruoso, tiene las habilidades de antimaleficio e indestructible.

Lion à crinière bouclée

{3}{G}{W} : Monstruosité 1. (Si cette créature n'est pas monstrueuse, mettez un marqueur +1/+1 sur elle et elle devient monstrueuse.)
Tant que le Lion à crinière bouclée est monstrueux, il a la défense talismanique et l'indestructible.

Leone Vellocriniera

{3}{G}{W}: Mostruosità 1. (Se questa creatura non è mostruosa, metti un segnalino +1/+1 su di essa e diventa mostruosa.)
Fintanto che il Leone Vellocriniera è mostruoso, ha anti-malocchio e indistruttibile.

羊毛鬣のライオン

{3}{G}{W}：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。）
羊毛鬣のライオンが怪物的であるかぎり、これは呪禁と破壊不能を持つ。

Leão da Juba Lanosa

{3}{G}{W}: Monstruosidade 1. (Se esta criatura não for monstruosa, coloque um marcador +1/+1 nela e ela se tornará monstruosa.)
Enquanto Leão da Juba Lanosa for monstruoso, ele terá resistência a magia e será indestrutível.

Fleecemane Lion

{3}{G}{W}: Monstrosity 1. (If this creature isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
As long as this creature is monstrous, it has hexproof and indestructible.

Pioneer Masters

Forgotten Realms Commander

{3}{G}{W}: Monstrosity 1. (If this creature isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
As long as Fleecemane Lion is monstrous, it has hexproof and indestructible.

Commander 2017

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.