It has a timid nature. If it is startled, the flames on its back burn more vigorously.
It is timid, and always curls it self up in a ball. If attacked, it flares up its back for protection.
It usually stays hunched over. If it is angry or surprised, it shoots flames out of its back.
The fire that spouts from its back burns hottest when it is angry. The flaring flames intimidate foes.
CYNDAQUIL protects itself by flaring up the flames on its back. The flames are vigorous if the POKéMON is angry. However, if it is tired, the flames splutter fitfully with incomplete combustion.
— Pokédex entries (via PokeAPI)