Yes while I understand that other species can be suitable companions they really are not ideal and honestly for a foal it doesn't even count... they need to learn to be a horse from a horse.
It will teach them the appropriate way to act around humans as well as all the good things about being a horse and will teach them basic obedience in a way a human just can't.
It's called "orphan foal syndrome" and while this foal has had his mother for the first part of his life to keep him solitary at this point would be nearly as bad.
While my horse was NOT an orphan and was very good with people- friendly but quiet and respectful he had no socialization with other horses and we had the hardest time getting him past that. I'm still not convinced he really "knows" as our small herd hasn't changed in years. Not at all aggressive just clueless.
There are just soooo many benefits to a companion and so many drawbacks to not having one. Ideally you would have a small herd with an older mare to be "mom". But not having that another horse really is a must. I would not consider another species (goat) to be suitable in this situation.
A 2 year old stud colt wouldn't be my first choice but it's better than nothing, however, as with adults you must make sure to introduce them slowly and carefully. I would not trust the 2 year old.