I think there must be a bug or something with them because I have a rainbow skeleton as well and I cant see it in equipper or preview so I havent put it on any of my horses yet so I wasnt sure if it worked on them or was just broken in Equipper and preview. 

Whats interesting is it doesnt even appear as a broken image, it just doesnt appear at all. 

I just put the rainbow skeleton on this one - https://horsefable.com/horse/1628555

It shows up for me, but it did NOT show up in the equipper until I actually saved it onto the horse.  I noticed that a long while ago when I first started playing, but haven't messed around with much decor outside of tack.

As for Asinnon's problem, what browser are you using and is it updated?  Outside of that, foal halters also don't show up until saved onto the horse, I believe blankets too.  So if you're only looking at the preview, you would not be able to see it until you used one of the uses on the item by saving/equipping it onto the horse.

Adding onto that - the rainbow paint will show up on the equipper page after hitting save, and also show on the horse's display page thereafter.

Adding halters/blankets onto foals ( Example: https://horsefable.com/horse/1666233 ) will not show on the page after hitting save, but will show up on the foal's actual display page.

Well im glad im not the only one with this problem. But i think i solved it, i tried a lot of different things. Trying different browsers, devices and then in the end all i was supposed to do was close horse image (on mobile) and again open it, so it would make things reset and now it shows! 

Ofc in the equipper it still dont show, but i hope they will fix it soon. 

Thanks everyone for responds<33