Is the a way to fix horse lag on Minecraft?

Once I get on my horse the server starts to lag like a mother $@#% Ther are only two ppl that play in the server and it runs just fine when I'm not on my horse. Is Ther any way to fix this. Oh and even when I have my horse on a lead the horse runs laggy but I run just fine.

Horse lag due to the amount of chunks being loaded, if you go back and then go forward again it will stop lagging but only until new chunks need to be loaded.

To make chunks load faster either dedicate more RAM to Minecraft (both client and server side), or you can install Optifine which optimises you Minecraft settings, allows you to enable and disable things to make it run faster. Other than that there's no way to fix it.

You can turn down the chunk generation from default 10 to like 4-6. While this will lessen the over all distance you can see. It drastically can reduce lag on a server.
A sever of 20-40 on 4-5 gig ram can run pretty smooth with the distance changed down just a bit, play with it.

This is of course if you can't give it more ram as Ducky suggested.