Half fat refers to a fat bike fork on a regular mountain bike. I don't think anyone markets a bike like that, its just a modification of a bike. I built my Surly Troll to able to run that way. I have a 135 rear disc spaced Moonlander fork on it and my normal front wheel is a rear wheel without a cassette. I have a fat bike so this setup hasn't got as much use as I anticipated. With a narrow rear tire you don't get the float of a fat bike, but you still get a stable handling front wheel. Before I built the troll I put the Moonlander fork on my touring bike for an ice race which was a lot of fun.