C.M.
She could have seen a snake outside, seen one on a cartoon (even a not scary one) or in a book. That being said, her fear of snakes is probably very real even though you may tell her that there are no snakes.
When my daughter makes comments about monsters or anything I think scares her, I just tell her in a happy voice "I'll tell him NO THANK YOU" if she says he's going to get her. Or I'll say "Oh the silly monster? He's just pretend." Another idea would be to get a spray water bottle and fill it with 'snake juice' which makes the snakes want to go home to their mommy's. Don't ignore it - talk to her about it and explain what dreams are - as she may be dreaming about them. Dreams may not be real but kids feel that they are.
Make sure she sleeps with a teddy bear or stuffed animal because 'snakes don't like them and won't come near you.'
Good luck!