Chubbies have a small chance of spawning from pre-set locations I believe. So if you revisit the areas that might have them repeatedly you might see one. They're extremely rare though. Two known spawn locations are:
- Right outside Hero's Pass, in the Eridian Blight. There's a Chubby Rakk. I've seen this one myself. I have also been in this area about 100 times looking for Donkey Mong and never saw a second one (or Donkey Mong) so the rate seems to be very very low.
- Right inside the Caustic Caverns there's a Chubby Varkid spawn location. I've gone through this area maybe 2-3 dozen times? And never seen it.
I wouldn't bother actively seeking these guys. As with most "rare" things in Borderlands 2, they're far too rare to make actively hunting them any fun. For the record I'm at over 150 hours game time and I've seen one. Allegedly the caustic caverns one is easy to make appear but I've never tried seeking it out.
Interestingly there's a lot of Chubby Rakk that spawn in an area in the Tourge DLC in a specific area (the only Rakk in the whole DLC, a mission takes you to the area). These Chubby Rakk seem to be a 100% spawn but don't drop the special look (or if they do it's an extremely low rate). I killed about 10 of them with no drops beyond ammo/cash.