The Buraq is a Tetrapod
Despite appearing at first glance to have six limbs (four legs and two wings), the Buraq is a tetrapod. It has four "true" limbs and two accessory pseudo-limbs.
If the Buraq is derived from an avian, its path to its current state is relatively clear. It has two wings and two (front) legs, which it can fold tightly over its body or extend, using normal joints. It's quite light weight for its size, with hollow bones and a relatively gracile body under a layer of down-like feathers, with flight feathers on the wings. The wings have traditional musculature, including a robust sternum.
Its rump extends significantly behind its (front) legs, causing it to be a bit arse-heavy. In the air this is countered by a large, fanned out tail that carries more lifting load than most other birds. This reduces aerodynamic stability, which increases the Buraq's agility in the air, at the cost of it being a bit twitchy -- that big fanned tail is always moving.
On the rump are two protuberances of erectile tissue, evolved from hemipenes. In flight, they are fully deflated, retracted within the down covering of the body. On the ground, they are normally engorged to be roughly as long as the (front) legs, and can support the rear of the Buraq's body in this state. When fully engorged they are twice as long as the front legs at maximum extension.