It comes from the logical fusion of the two words and can be used to mean anything from:
Rats, Mice, Rabbits, Bats, Squirrels, Chipmunks and most infantile mammels.
Everything smaller than a typical house cat can be considered a "Smammel"
Guy 2: Well, we are big strong men, we can survive
Guy 1: You know that there are nothing other than Smammels in this part of the woods? What will we eat?