Yeah, Elegon is highly likely (barring pet store/mountain o' mounts v.2) because since the August drake has the word Thundering in the title, that implies lightning effects like the guild reward. Also the word August implies summery colors which, imo, leans towards the gold or reddish one. Now watch the Celestial one have a .000001% raid drop...Bah. I do hate those lotto mounts.
*on 186 Blue Proto runs and counting*
Ah well, if the August Celestials mount is cool that'll make up for it!
EDIT: Hmm, just noticed its icon is the yellow serpent (IE the Golden). Calling it now, Gold dragon with lightning! ...I hope they change the color of the lightning effects if they do that though, bright blue lightning on gold sounds almost Heart of the Aspects levels of eye searing.