Horses are found in plain biomes, and you can spawn them with the spawn egg in creative mode...
That is not a bug, that is just what occurred to you while playing Minecraft.
Not the trigger sound effect... If you walk on a solid block, like wood, it makes a certain sound effect. It is not supposed to happen with the pressure plates!
Please pay attention to what I was saying!
Note that I was using the 1.6 Prerelease!
I would like to correct you, Kumasasa if I may, that horses spawn only in plain biomes that were generated in the horse snapshot or later...