Final version of Swift Foot from the Feats of Friendship comic.
 
>>2101216s
 
Headcanon begins here:
 
The standard mare body is for 18-to-30-year-old adults.  

 
The slender, “beanpole” teenage AJ type body seems to me to be roughly 12-14 years old.  
>>1268013t
 
While the filly model seems to be 8-11 years old.  

 
I believe all the friendship students are US high-school age, putting them at 15-16. Sandbar’s body and limbs have similar proportions to an adult mare body, although it is a bit more squarish. He’s definitely smaller than an adult stallion. With that in mind, I wanted a 15-16 filly body, so I took the adult mare proportions and made the limbs a little bit more slender, partway between adult mare and teenage AJ. The belly is also a little less rounded than an adult’s.
 
Finally, I kinda like the ear style that Fleecs is using, which seems to be a hybrid of show and movie styles that IMO looks better than both. It captures the 3d aspect of the movie ears while remaining more show-faithful.
 
Does it work?
 
Flirty version: