A type of dragon in my world, simply named the Thick-horn, has a well-known mechanism to use to defend itself despite its clumsiness. They are freakishly large, tend to be slow, and find it hard to fly without leaping off of some sort of high place, despite their bird-like bones. Their kind tends to fight against each other and other species often, requiring them to use this ability.
They can set themselves on fire, everyone.
But even I'm not sure how this biological phenomenon works, and I want to get to the bottom of it. Simply, the dragons sparks fire in their own fur, which spreads but does not harm them. They are intelligent, but I don't see how they would get their hands on extremely rare and fancy materials due to their... animal status.
Less ramble, more dragons! How do these dragons set their bodies on fire like this, without harming themselves?
(Notes: Previously described: some sort of secretion through the skin, maybe take some inspiration from this.)