Dawn is the default name of a unicorn foal companion pet available on Solomon's General Store, for 342 RuneCoins for members and 380 RuneCoins for free-to-play players. It does not have any growth stages.
Interacting with the pet prompts the player if they'd like their pet to perform its emote. The emote consists of the unicorn stomping its hooves on the ground, creating a small green patch of grass. It will then start to fly around in a circle.
Dawn may be recoloured; doing so changes the colour of her mane, tail, and wings.