If I kill my horses will it spawn more horses in Minecraft?

I found some horses on my server, but there not the type I want. Is there a way to get more to spawn? like if I were to kill my original spawned horses will more horses spawn?

It is very unlikely for passive mobs to spawn in already generated chunks. It could happen, but it's just very unlikely to. What you should do is explore new chunks and find some more, then bring them to spawn and breed them so that there will be more.

In addition to what has been said, @SevenSidedDie pointed out that you will need to kill all the animals in a large radius for the slim chance that a new horse will spawn.