What do dragon riders give their dragons to breathe fire?

Answer 1

Dragons breathe fire, ice shards, toxic gas, acid, lightning bolts, pure light, pure darkness, and lasers.
Dragons traditionally breathe fire, including Western-Asian dragon crossbreed hybrids. I know a dragon who can breathe ice, though, and full Asian dragons are said to breathe mist.

Answer 2

they have a gland full of flammable gases that releases the gases up the dragons throat, which ignites when they reach the back of it's mouth-which happens to be very hot

The ancestral dragons had 5 lungs. In the 'higher' members of the class the dorsal lung has become a specialized organ that secretes and stores a highly nitrated 'phlegm'. When the dragon expels this phlegm it become an aerosol in the upper throat where all the larynx' converge.

Now Dragons are very tolerant to heavy metal poisoning, will even seek-out such to ingest. (Which explains their inclination to raiding Dwarvian kingdoms) The effects of this are very evident in the colourful scale designs. Another effect is throat plates with a high heavy metal presence that acts as a catalyst to the violent decomposition of the highly oxygenated nitrated in the phlegm. The result is a flame.

Answer 3

Assuming that legends are true, then there were certain dragons that breathed fire.