Image imported from derpibooru 
Original Description:
 
So, long story short in the RP i’ve been playing for the past…10 ish years or so we use traditional elemental magic to supplement our ponies’ powers (You know the ones, earth wind fire water, light dark…) with one additional element tied to Moonlight (Those who played Boktai know why)
 
Each of our main player characters has some ties to one or more of these elements, with Moonbeam being tied to Moonlight (duh…it’s in her name), each mage after reaching at least level 6 in their studies gets access to a spell called “Elemental Form” that boosts their power output at the cost of draining mana over time. This is Moonbeam’s elemental form.
 
Those of you who know about Shadowbeam (her twin sister) will note the eye coloration and the spikey looking mane simillarities between the two in this form. That’s because Shadowbeam has never been depicted outside elemental form.
 
And no, she’s not really a “blank flank”, her species just don’t get cutie marks to begin with.