The draconequus to the left is his sister, Symphony, the draconequus of harmony. Back before Discord's reformation, she didn't exactly have the best relationship with him. In fact, they fought together at times, but with her brother being too skilled and adept with his cosmos-bending magic, she sent him back to his home realm every time in an attempt to "ground" him, to say the least. As for her body parts, my headcanon for draconequui is that while they always assume a form similar to eastern or western dragons, their body parts are thematically determined by what conceptual aspect of creation they embody. For instance, Discord's burton-esque, mishmashed appearance corresponds with his alignment with sense of chaos, and Symphony's more blended, prettier looks symbolize harmony.
Here's the short story I made for her on DA:
> Symphony, the draconequus princess of harmony, didn't exactly have the best ties with her brother. Her duties as an angelic interdimensional guardian involved her saving entire worlds from threats of near-cosmic proportions, and Discord, the lord of chaos, was no different. In ancient times, their battle shook the land until she had whisked him back into his home dimension, leaving her to undo all the damages he caused.
> Many years later, she would catch wind that the purveyor of pandemonium seemed to have a change of heart. That her brother had turned over a new leaf to make new friends. Practically confused, she decided to investigate out of concern that he was up to another one of his tricks, but eventually, she started to notice the care that he had hidden deep down within his soul for so long. The more Discord had welcomed her with open arms, the more she started to see what her seemingly mismatched horse leg symbolized. Perhaps both chaos and harmony can coexist to create something magical.