"I'm starting to suspect that there are no legitimately obtainable, truly rare vehicles, merely 'uncommon' ones.
The Fixter would like a word with you. It's Fixed-Gear bicycle. I doubt a single person even has one online, or maybe less than 10, as anyone who has seen one is probably a random dumb person who'll write it off as an ordinary bike. It's in there somewhere. But how rare is it exactly?
Well, for starters it doesn't even have GTA Wiki page. Second, in SP the way I knew to obtain it was a glitch were if you bought the Endurex Race Bike as Michael a Fixter would be delivered instead. I haven't checked if the bug still occurs. But it IS the rarest Vehicle in GTA Online without a doubt. And if that glitch no longer occurs, the rarest vehicle in SP too. Originally I actually found the glitch annoying, as it wouldn't allow me to obtain the Tri-Cycles bike.
What does it look like? It's all black, looks like a classic style bike, Vintage style, Yellow-lined wheels, it's about as big as the Race Bikes, it has the Curved handle-bars too. Here's some pics I could find, there's a youtube video too:
Put it this way, the Glitch obviously wasn't intended as it went like this (As Michael):
Whippet Race Bike = Whippet Race Bike
Endurex Race Bike = Fixter
Tri-Cycles Race Bike = Endurex Race Bike
If that so-called "Bug" wasn't intended. Then the Fixter MUST be in the actual game world somewhere. Intrigued?