Twilight Sparkle

Size - Average, long horn / wings

Build - Average

Fluff - Average

Eyes - Average, dark

Mane - Straight but slightly frizzed

Book horse I think is a mainstay (pun) source for all things average IMO, except her eyes and hair. Twilight's extremely busy lifestyle and constant stress I feel warrants her to have darkened eyes of insomnia. This is a pony who could sleep for 12 hours and still not wake up rested. I also don't think she'd spent a lot of time on her appearance, letting her mane and coat remain brushed at minimum.