Egg description - grayish purple shell with two wings and a long slim tail poking free. Lightning courses over it in sporadic bursts. Particularly from the tail itself oddly enough.
Hatchling description - The strange creature looks a bit like the playful Featherdrake. With a few key differences. Namely it bears an actual beak upon its face and its coloration looks like a mix between clouds colored with the setting sun and a storm rolling in. Much like the Drakes from whence half of its genetics came it is a bit clumsy and fumblefooted at this stage in life. With the added trouble of occasionally emitting sparks of lightning unless its Magi takes care to avoid startling it. Its adorably fuzzy appearance leads many unwary and eager young things to playfully greet a possible new friend; only to be shocked quite literally by its less than social response.
Adult - Thankfully with age and experience this far more gryphon-like drake is less likely to shock its Magi or those it is introduced to. Another upside is this new creature seems to have stayed relatively small, not growing to the titanic proportions Thunderbirds are known for. But still easily trainable as a steed if need be, if not a guard beast of its Magi's dwelling.
General note on the species:
These odd entities seem to personify the sort of partner that will do anything to help their chosen Magi. A prime example being their explosive anger if an enemy were to harm the Magi they deigned to bond with. Resulting in a persistent hunter that doesn't let a little thing like distance or time dull their righteous fury. Where they will leave the rest of their Magi's partners to watch over their human these thunderous hybrids will vengefully hunt till they find the ones who hurt their Magi and make them regret ever crossing paths with one who had been chosen by a creature such as themselves.
Male divergence - Males retain the 'sunset' coloration on their belly and wing feathers after they hit adulthood. Personality-wise the male Sparking Featherdrake is by far the more violent as well once angered; insofar as they will take the most direct route possible to make an enemy rue the decision to hurt someone these entities had grown to respect.
Female divergence - Unlike their male counterpart females darken to shades similar to their thunderbird cousins. Likely a magi-thought up survival tactic to allow the females to shield their nest in the dark tops of pines where they feel drawn to nest and lay their eggs in the mating season. A female Sparking Featherdrake in comparison to its Male counterpart is much more conniving and sneaky when it comes to making an enemy regret messing with its Magi; who it perceives as part of its Flock. They as a result will make life progressively more miserable for a target of their Ire till finally deciding to strike down the 'heart' of the problem.