They diet of a Unicorn can only be speculated upon. There are two conflicting theories as to what one eats, or what it does not eat, as you will see.
The first hypothesis is that it eats as a normal horse would, grass, oats, hay and the like. There have been sightings reported in legends in forests, head to the floor, cropping intently at the grass. This behaviour could also be interpreted perhaps that the animal eats just because it is expected to be seen doing so, or perhaps there is some other purpose. It is not known.
The second, although seeming far-fetched and wild, is widely accepted. It is believed that the animal does not need to eat at all, being an animal of the Gods. It simply absorbs the solar power of the sun, drawing the energy directly from it. Many believe that the horn plays a major role in this process.