this might be a bit of an unconventional idea, but i can't help but think of the Breyer mustang/Fireheart model.
https://www.identifyyourbreyer.com/identify/MustangStallion.htm

i don't mean something exactly like it but i just love that sort of fighting posture with 3 or all 4 legs on the ground, with the mane and tail flying up from the movement. it's really unique compared to rearing or running. the need to fight off animals and other horses sets mustangs apart from many of the breeds we have, so i'd love to see something in the spirit of how mustangs endure on despite everything.
some examples that i think are similar to what i mean:

fingers crossed the images will work 😅

I LOVE forgottenlands idea. That's the Mustang spirit!

Ooh, great idea, forgottenland! I love the second image especially!

I love those I really like the rearing and three legs on the ground and I agree I would love to see tail movement.

Love forgotten's idea!

I was thinking something at least like a leg off the ground like pawing at the air, challenging kind of. 

Like this, this, this, or even this. 

I dont know if I want another kind of rear. Its common for mustangs and what we have now is like a half rear thing that looks like its a chubby cute my little pony almost haha 

I agree with forgottenland

aaa love those examples Maki!! especially the 1st and 3rd one!

Maki and Forgotten 

I love both of your 2nd pictures!

This aligns with Forgotten and Maki, but I always think of Elisa Wallace's work with mustangs and how versatile they can be. Her mustang Hwin has a Breyer model that has a really lovely silhouette with a foreleg off the ground and a proud head/neck carriage.