is it kinda crunchy and frizzy? this happens sometimes (my TTM Celestia has it, I weep) and the G4s seem to have a particular problem with it on their tails. :I
this is the way other people have dealt with it; I personally have not tried to de-crunch a pony yet (although my poor Celestia is making me consider buying a flat-iron):
1 - smother the hair in conditioner, then leave it in there for like a week. or two. some people have had better luck with hair-relaxing shampoos and conditioners; some people haven’t, so it’s up to you if you want to try it. apparently that kind of conditioner can damage color-change hair, so if you’ve got a G4 custom or an earlier-gen pony make sure it doesn’t have color-change hair first!
2 - mix equal parts of fabric softener and water; drench the pony’s hair in it and then brush until smooth. this is really a detangling method (I’ve done it on my childhood dolls and it’s MAGIC) but sometimes it also helps with frizz!
disclaimer: the following two methods involve boiling water and hair irons, both of which get burningly hot and are potentially dangerous if done incorrectly (I’m a Certified Adult™ and I’ve burned myself before by not being careful). DO NOT do these by yourself if you don’t have experience with using a stovetop or an iron; get a parent/guardian/friend who does to help you.
3 - boil some water, hold the pony over the pot as it boils, and brush out the hair in the steam. if that isn’t enough, remove the pot from the stove and let it cool for a minute, so it’s still hot but no longer boiling. dip the hair into the water and brush it out, repeat until smooth, then add conditioner and wash as normal.
4 - this is a much more reliable method, but it involves using a hair flat-iron, so seriously, be careful. you don’t want to burn yourself OR melt your pony’s hair. also, do NOT use this on ponies with tinsel hair; tinsel will melt much more easily than the nylon and afaik no one knows of a good way to protect it!
first: rinse the hair, put conditioner in but don’t wash it out yet (you don’t need to drown it in conditioner, but you can be a little more liberal with it than usual). once the iron is heated up, QUICKLY (so as not to melt the hair) run the hair through it. you can do this more than once, but let the hair cool down inbetween swipes to be safe. once that’s done, leave the conditioner in for a couple hours and then wash out as normal. here’s a video tutorial on it by yummieyumyum87, pay attention to how quickly she uses the iron!
(for the record I would not recommend using a clothes iron on pony hair unless you really know what you’re doing. always put conditioner in the hair to help prevent melting, and test it on a small patch of hair first.)
if none of that works, and sometimes it doesn’t…your best option is probably just to try and hide it. :( you can put the hair in a really tight curl, or at least curl it in a way so that the frizzy part doesn’t show.
eta: okay I bit the bullet and bought a cheap flat-iron, tried it on my Celestia. already I can say be REALLY careful and let the hair cool 100% completely between swipes…I accidentally melted the ends of some of her hair. :’D waiting for it to dry now and we’ll see if it helped the frizz at all.