Chillerpillar
Snow Creature — Insect
: Monstrosity 2. (If this creature isn't monstrous, put two +1/+1 counters on it and it becomes monstrous. can be paid with one mana from a snow permanent.)
As long as Chillerpillar is monstrous, it has flying.
From its cocoon, a flurry takes flight.
Illustrated by
Modern Horizons#43CommonEnglishLegality
Rulings
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, loses its abilities, or loses its +1/+1 counters, it will continue to be monstrous.
Chillerpillar gaining flying after it becomes blocked won’t remove the blocking creature from combat or cause Chillerpillar to become unblocked.