It usually canters casually in the fields and plains. But once a Rapidash turns serious, its fiery manes flare and blaze as it gallops its way up to 150 mph.
Biology
Physiology
(from Bulbapedia)
Rapidash is an equine Pokémon with four slender legs and black hooves. Its body is cream-colored, and it has a horn on its forehead. It also has small red eyes and pointed ears with red insides. Red and orange flames stream from neck, part of its back, and its fetlocks. Flames also form this Pokémon's tail.
Rapidash is a very agile Pokémon. It runs at about 150 mph (240 km/h), chasing everything that moves in hopes of racing it. With incredible acceleration, it reaches its top speed in only ten steps. At full gallop, its hooves barely touch the ground. The faster Rapidash go, the longer the swaying flames of its mane will become. At an all-out gallop, its blazing mane sparkles, a feature which is said to enhance its beautiful appearance. Rapidash lives in grasslands, but it can also be seen on mountains in Pokémon Snap.
(from Bulbapedia)
Rapidash's horn shows it may have been inspired by unicorns, but it more closely resembles a regular horse. Rapidash and its pre-evolution may also be based on fiery steeds in mythology, such as those of the Greek god Helios, Skinfaxi of Norse mythology, and the horses of the Ashvins in Hindu myth.
Name origin
(from Bulbapedia)
Rapidash may be a combination of rapid and dash, referring to the high speed at which it runs. It could also be a combination of rapid and ash (referencing its Fire type).