If you have problems with muggers a horn won't do a blasted (pun intended) thing. Weapon? really like what? A gun? ok, a mugger is hiding behind some brush waiting for some moron to come riding by on his bike, he jumps out and suddenly and unexpectedly knocks you off the bike, just how fast do you think you're going to get your gun to prevent that? you're not going to get any chance of getting your gun out, you're laying on the ground probably in pain, you're disoriented, scared, and trying to figure out where your gun is, you finally find but the mugger sees you trying to get something so he starts to beat the **** out of you, finds the gun and shoots you with it.
I love it when I hear people carrying guns with them while riding, how ignorant they are. Even if the gun is in a ***** bag you won't have the time do get it out not alone aim to fire it. And if it's a car that hits you you're laying on the ground injured, even if you are able to retrieve the gun the car is gone.
The only chance you have against a mugger, this won't work for a car attack, is to carry some pepper spray, at least if you get it in the general direction some may hit the eyes of the attacker. But you have to place the spray someplace where you can get it fast like on a clip on your shorts except most shorts won't handle a clip because they don't use belts, so you may have to attach it alongside a water bottle cage, but NOT inside a bag. The only other option, to go along with the spray, is to take Muay Thai, or Brazilian Jiu-Jitsu, or Krav Maga, or GoJu Ryu (either 4 are good just chose the one that has a class closest to you) for self defense, but you need to take it for a long time so it becomes second nature, how long? at least 10 years to learn all the facets to it. Don't waste time learning various forms of weapons in martial arts, none of the weapon **** they teach you will you have on you, or find one laying near you when you need it.
I took Goju for a long time but only recently did I find out that Krav Maga uses a lot of aspects from Goju. Most people either don't know about Goju or overlook it as not being in the top.