I'm using the latest version (0.4.0c) and this issue is bugging me- I think it's been happening the whole time, but the new penis clipping technique makes it look a lot worse:
The horse's anus doesn't conform properly to a shaft that's being inserted, it sort of forms a 'halo' around it. This results in ugly clipping issues (at least if you're on a slower computer). The effect is a lot more noticeable with the light skinned horse and his glowy insides.
Screen capture example:https://www.dropbox.com/s/dxoob55fnamic6k/horse_halo.gif?dl=0