I must have opened at least 2k glory chests and 100+ gem chests, but still have not gotten Mongo.
I am only missing the achievement that requires unlocking him as a troop. Is he currently still available in-game to be pulled from chests?
As far as I am aware he is. I need a couple more to make him a mythic.
He definitely is, I got another one last week 
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My impression is that Mongo is one of the slightly harder Epic troops to get. After the Devs gifted all of us with one copy, it took a while before I obtained another from glory chests.
Then again, it also took a long time for me to get an initial copy of Green Seer. So some Epics may indeed be slightly rarer in the pool. But yes, they are available to obtain. 
Now if only I could get my first Summer Imp for my console account… without waiting another entire year…! 
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