Couple art: the pones are together/in some form of romantic/platonic relationship. (Usually 2 goopy pones)
Companion: kind of like a pet, or a non-goopy pone animal
I guess if your goopy pone has a friend or romantically involved with another like horse type breed, it would count more towards couple art.
Also! The mane! Its common! You can totally add a mane for free. (I just typically draw them without b/c I like how they look in my style without one xD)
But yes, adding goo where a horse would have long hair; mane/tail/legs is totally cool beans. Also changing the breed, like if you wanted like a pony sized goopy pone or a draft. I pretty much give free will of that ;u; I just ask the goo all pretty much matches for the most part. (like if the tail was like. purple, blue, and green, the mane would have at least 2 of those)