Transformation triggered by creature age or size

Kulshedra

The serpent form called bolla — the name survives among the kulshedra's own variations — has faceted silver eyes, four legs, a long snaky body, and small wings; with the years it matures into the kulshedra proper, growing horns and nine tongues, its wings enlarging, and gaining the breath of fire.

Nekomata

This metamorphosis is often accompanied by an increase in size, with some tales describing Nekomata growing to the size of large dogs or even humans.