My truck came with one. I think its an air horn style, has a little compressor mounted under there. The thing is freaking loud. Sounds like a train.
My girlfriend honked it the other day when i was walking in front of it, and i jumped a foot.
The only thing that sucks, is when i lock the doors, and the horn honks to let you know you locked them, it doesnt work. I guess because the trucks not running and it doesnt have air