"I had gazed on him while unfinished; he was ugly then; but when those muscles and joints were rendered capable of motion, it became a thing such as even Dante could not have conceived (36)." 

-Dante deli Alighieri was a major Italian poet of the Middle Ages and is the author of the poem, the Divine Comedy, which is considered one of the greatest works of world literature. The poem describes Dante's travels through Hell, Purgatory and Paradise in detailed, imaginative ways that represent, at a deeper level, the soul's journey towards God. The poem shows that Dante was very insightful and imaginative poet that could conceive many possibilities in life. Shelley uses his name in Frankenstein to show how horrid and unbelievable the creation of the monster was. She narrates Victor's thoughts and how horrified he was that he thinks even Dante, who can create such wild stories, would not be able to imagine such a situation as his. She may also have mentioned Dante because the Divine Comedy represents journeys through life and the afterlife towards God and the change that one goes through, which is what the monster and Victor are both doing throughout the story. 

http://en.wikipedia.org/wiki/Divine_Comedy