What I expected to happen was...:
Horse reins have no blue, gold, or gray floaty bits

What actually happened was...:
When saddled and wearing armor, a small piece of the armor appears floating at the end of the reins. Basically it renders the reins box as if it had more than 0 width, but only for the armor.

Steps to Reproduce:
1. Tame a horse

2. Saddle it

3. Put armor on it

4. Climb on and look at the reins, towards the bottom of the screen

I fixed this issue in my own mod by calling addBox on the horse rein modelrenderers without the scale argument, just the 6 floats. There's probably a better solution though.