What is an example of a man vs the supernatural conflict in Fahrenheit 451? I thought it might have been Montag vs the hound but that would be man vs technology.
I think you're on the right track with the Hound. However, I would suggest that you prioritize your teacher's definitions of the term "supernatural" over my advice. Without knowing your teacher or the assignment, I can't confidently say whether they're looking for an interpretation of the term, or for a specific example which demonstrates that you either do or do not understand the concept.
You can make a pretty good case for the Hound being supernatural by focusing on how it is characterized; I feel like the Hound embodies a strange contrast between what should be, and what is. It is suggested that it has various qualities that it should not, like emotions, such as when it growls at Montag. Beatty suggests that this is merely a chance or circumstance, with the Hound being momentarily confused between Montag and some former target, but then one would wonder why the Hound is capable of growling at all, when its express purpose is to track and subdue its targets. The growling is never truly explained (I initially thought that the Hound might have been programmed to sense some kind of chemical change in people who were having dissident thoughts), so I think this would be an example of an infusion of supernatural qualities into what should be, by all definitions, the exact opposite of the supernatural (i.e. technology).
I think a more straightforward interpretation of supernatural conflict is contained in Faber's brief mention of the personification of Jesus in this society. According to Faber, Jesus is "sugar-coated" and makes veiled references to various consumer products; he wonders if God would still recognize him. This isn't so much a direct conflict between person and supernatural as it is the outcome of that conflict; humanity has constrained and controlled the supernatural, and removed all the elements which made it supernatural in the first place. We would never expect a supernatural being to endorse a consumer product. Thus this shows that even the things which are supposedly beyond human comprehension have been subverted and reformed to support the central narrative of this society, namely, mindless consumerism.