I'm really convinced Black Horse is Sanji's opponent EoS.
Not only both have Black as their epithet and not only both are strongest outside the Luffy and Monster Trio / Fleet Admiral and Admiral Trio.
But also not coincidence the animal is Horse.
When you hear tiger, you imagine their strength, their claw, and the power of their jaw. Just like Zoro who using his jaw to perform the Santoryu.
Or when you hear bull, you imagine the strength of their horn, just like Yamao who also has horn.
But when you hear horse, you imagine how fast horse can run, using their Legs. Not coincidence his opponent not only has Leg as his epithet but also uses legs as his main choice of weapon.