My advice is to ask your DM sooner rather than later. If you spring it on him right before you want to do it, you're more likely to get a "no" because the DM won't have thought it over and his initial reaction will likely be colored by the fact that he has a whole lot of different stuff planned and a talking Mongo wasn't part of any contingencies he'd planned for. If you give him time to think about it he might spot some potential story ideas that would make him more likely to say yes - and also give him time to get over any "talking animals are inherently silly" reactions he may have.
Once you have DM buy-in it really doesn't matter what those of us here in the peanut gallery say about the "rules as written" or not. And without DM buy-in the same is true. It's the kind of question that is all about worldbuilding and the story of the party rather than combat balance, and so what your table decides to do may be very different from what my table decides to do. (Personally I'd probably allow it so long as the players involved had an idea of where they wanted it to go and weren't expecting me to come up with some elaborate back-story for Mongo myself. I'd want to know what that backstory was beforehand too - both to tweak it a bit if it didn't fit in with any of the gameworld secrets I was keeping from them and to see if they'd stuck in any story hooks for me to hang a game session or two on. But that's me - different DMs are going to have different attitudes to stuff like that.)
