Agree with using water to dilute the paint- make it VERY THIN - it may take you 3 or so layers to build up the color- but it avoids the brush lines and clumping.
Honestly I would probably try removing the paint and trying again. If the custom is recent enough, you may actually be able to get a lot off with some soap and water and then acetone for the stubborn parts. I noticed you got some on the mane and sometimes if the paint hasn't been there long it will still wash off. But otherwise, acetone. You can get it SUPER cheap at a hardware store. !
I know thin careful layers are labor intensive- which is why some people break down and buy an airbrush (which also require layers)- but the results are worth it!
You might try a color that is more opaque- white actually requires more layers than most.