The trail start looks ugly

[MCMic]

The start of the trail is too sharp and is beneath the pony without any particular reason.
It should start behind the back legs in a nicer way so that we don't see this sharp start.

[ginkgo]

This is true.
I've considered improving on the trail visuals as well recently.

What would be easy to do and would really improve the visuals would be if we added textures to the light-rails. Maybe some aurora borealis-like texture would be good for this. A texture like this should be relatively easy to generate, so there would be no copyright issues.
The start of the trail can be improved by slowly scaling the height at the start instead of the sharp beginning.

This would improve things a lot, but one problem that would still remain is that the trails have no physical depth. I.e. you don't see your own trail when you run straight.

My original idea back when I started making the game was to have these trails to be made out of flowers with flower objects/billboards spawning around the trails. I think this could look quite nice.

[MCMic]

Maybe the trail could have glow and blurr on it to look better.

[ginkgo]

Glow/Blur would be pretty nice to have.

The best way to implement this would be to render an additional layer with trail colors and blurring that. The most efficient way to do this would be if you used Multiple Render Targets in the shaders. I'm just not sure if the Mesa support for this is there yet.