Here's how Mary Shelley describes it in the novel:
* "I had desired it with an ardour that far exceeded moderation; but now that I had finished, the beauty of the dream vanished, and breathless horror and disgust filled my heart." (Chapter 4)
* "His yellow skin scarcely covered the work of muscles and arteries beneath; his hair was of a lustrous black, and flowing; his teeth of a pearly whiteness; but these luxuriances only formed a more horrid contrast with his watery eyes, that seemed almost of the same colour as the dun-white sockets in which they were set, his shrivelled complexion and straight black lips." (Chapter 4)
Frankenstein is horrified by the creature's appearance, which is far from the beautiful and noble being he had envisioned. He is repulsed by its grotesque features and the unnaturalness of its existence. This initial reaction of fear and disgust sets the stage for the tragic consequences that follow.