So, I've always wondered how you artists apply that outline on your horses. Like, what brush and size, as well as what type of blending and such

When you say outline, what do you mean specifically?

I mean that line that you put around the horse, often outlining muscles and the horses cheek.

I used brush size 1. It's easier if you have a program with pressure sensitivy as the lines are able to taper off and adjust to how hard you're pressing. I usually don't smudge those ones.