It's fairly clearly a cartoon bear going through hibernation.
It's very physically strong, eats a lot, then goes to sleep. Additionally, Munchlax is found on honey trees - bears are stereotypically very fond of honey, a la Winnie the Pooh.
It can be seen as having elements of a fat cat as well, due to the pointed ears and the fact it's personality is a super strong Garfield. The only hole in this concept is the lack of a prominent tail.
Given that Snorlax is frequently seen standing up to fight, I think it's pretty clear it's mostly based on a big hibernating cartoon bear.
