I found and killed him twice in my game
It's easter egg challenge like easter egg achievements for Donkey Mong and Chubby to killed them. Random and rarest to find. You will find them in no time while playing game. If some of you can't find them in 1st playthrough then do it in 2nd playthrough, join other player or friend game or so depend your luck to find them.
Tip for Donkey Mong, he is random in same spot of course many forums and here mention it so if you can't find him first time then quit to main menu then reload and go find him if nothing do it again until you find him and kill him off. I got achievement hour ago, I have few times reload to find him. So chubby and Jenkins is natural progress for me because found them regulars in random places.
Quick short note as I found them: (maybe help you to find)
Donkey Mong - Alway random in same spot in SW Eridium Blight. There and here explains already and myself too. If can not find him then keep doing reload game until you win yourself to find and kill off in same spot.
Jenkins - Found him in chest right before big Goliath in wide open area in The Fringe (part of side mission where you have killed big Goliath with big chest on his back).
- Another Jenkins I found and killed him in Lynchwood, I found him as he wander around nearby Bandits in almost first area when you arrived wide open area after train station with fast travel and vendors anyway found him in alleyway while I killing bandit enemies and saw Jenkins and killed him off for more loots.
I yet didn't saw Jenkins in either main areas you lots mention it. I found him as I explain above it's Lynchwood and The Fringe is another chance to find him. He just like luck based for anyone find him.
Chubby enemies - Found some of them in my game. I knew and remember killed Chubby Spiderant in cave in Frostburn in early game and found few Chubby Spiderants in The Dust by doing missions and leveling up in natural progress. Also found Chubby Varkid in Caustic Caverns. Also there notice bit more numbers then rarest chubby enemies to find in late playthrough (I am in late playthrough and at Eridium Blight moment). :P
All those are random for you find and you will have them in no time by playing natural progressions.
Gearbox Software loved rarest, secret and random things for people to find.
Enjoy Easter hunters.