This guy is portrayed more like an animal than a human being in this movie, because he's a huge, terrifying strongman. Or as Jim tells Bart in the early going,
"Well, Mongo ain't exactly a 'who.' He's more of a 'what.'"
People feels this way because Mongo has the strength of ten men and will basically do whatever he's told. In this sense, he's the perfect goon. But when Sheriff Bart outsmarts Mongo with an exploding box of candy, Mongo can't help but think,
"Sheriff first man ever whip Mongo. Mongo impressed, have deep feelings for Sheriff Bart."
So even though Mongo might seem like a big gorilla on the surface, he's actually a loyal softie at heart. When asked what he knows about Hedley Lamarr's plans for the railroad, Mongo shows us his poetic side by saying,
"Don't know. Mongo only pawn in game of life."
Aww. So even though Mongo might seem like an unthinking brute, we find out that he actually knows his place in life quite clearly and even feels a little sad about it. Then again, Mel Brooks plays all of this for laughs… so don't bother feeling too bad for old Mongo. He ends up fighting on the winning side anyway.