Symbolism, Imagery, Allegory
Every time he's at Big Sur, Jack mentions the mule he names "Alf the Sacred Burro." Alf is a reminder of the spirituality and primitiveness of Big Sur. This wilderness is like a Californian version of the "Garden of Eden," and Jack mentions Adam and Eve twice while at Big Sur. Remember that Jack vows not to kill any animals – even rats – while staying at Monsanto's cabin. He makes a point to feed and care for them all, including Alf, in his attempt to isolate himself from people by taking refuge in the natural world.