As an author of high fantasy, you can write whatever you want with no rules. Right?
One key aspect for becoming a great fantasy writer is to keep to the rules. What rules am I referring to? Why, the ones you create as you establish your own realms, kingdoms, worlds and universes.
For example, in my first book series, The Incarn Saga, I created a race of beings called Theriomorphs who have the ability to transform into an animal form. I said they are born with this capability, cannot choose their animal and can only transform into one kind of creature. Once those rules were established, I had to keep true to them. This means I cannot have a character changing from human to dog to horse to mouse to elephant. That would counter my own rule and thus make the whole story line and world less believable. Once you make a rule, whatever it may be, you have to stand by it for the duration of your story, book or series.
Well, what about facts? When you write fantasy, certainly that means there are no true facts?
A fantasy writer can choose to incorporate facts into their fiction pieces. This makes a fabricated world seem more real and thus believable which pulls the reader into the story. In The Incarn Saga, I use a lot of facts about animals and animal behavior when dealing with the Theriomorphs. There is mention of rutting season and timing of antler shedding in reference to Talos, who can change into a white-tailed deer. Terminology like chuffing is used when Lluava is a white tiger or keckering, which pertains to the sound a badger makes.
Yet the nice thing is that since my work is fiction, Lluava’s tiger form can do things that a real tiger cannot—like purr. Yes. Tigers cannot purr; they chuff. The villainous Raiders, though Viking inspired, do not keep 100% true to actual facts pertaining to the Viking peoples. For instance, their helmets are often horned which is a modern misconception as actual Vikings never wore horned helmets.
Overall, when creating a fantasy world, you can pull in actual facts; however, if you make up rules, you need to follow them. Yet you have the amazing ability to choose what rules and facts (if any) you wish to incorporate.
You have the power. Now write!