Is this a race for people with giant butts? Not quite. A fatass race is essentially an organized “unofficial” race set up by one or a few dedicated ultrarunners. There's usually no perks (shorts, medals, etc.), course marshals (people that keep you on the course), aid stations or other forms of support, or even a timing mechanism. It's usually more like a training run than a race.
So why would you want to run a fatass?
And they tend to be super cool. The people that show up for fatass races are running for the pure love of running and the camaraderie of their fellow runners. I would highly encourage any new ultrarunner to hang out at fatass races.
However, I would not recommend a fatass as a first race. The lack of support usually requires runners to carry all their food and water, which adds a fairly difficult obstacle to an already difficult undertaking. Also, the lack of course markings may make the actual course navigation difficult. Again, it's another variable that a brand new ultrarunner shouldn't have to worry about.