There is no origin story for Pooka. Pooka appears in stories, and admits of no explanation.
Pooka's shapechanging ability is in fact the ability to hide, for a time, its fae nature. All fae have control over their Dreamstuff form; only Pooka can make itself appear to be something other than Pooka.
When Pooka is scarred by Ironfell, it loses, in part, this ability to disguise its nature. Where there was Pooka before the Sundering, after it there are many Pooka, each of which can take on only a single disguise. That is, each Pooka can appear to have only one other nature. After the Shattering and Pooka's adoption of the Changeling Way, this ability manifests in the ability to change their Dreamstuff form, whether fluid or petrified. This is a deep power, for in all other circumstances the influence of the Dreaming on Ironfell is solely to unpetrify Dreamstuff.