All dragons must be fed when owned, and depending upon the substance, you will have to feed them every ten minutes, too every in game week. If not fed, they become feral, and hostile. If a non-combo dragon, you get an egg, but all combination dragons are sterile, and you do not get any egg back. The substance needs, and how much in a given time, is detailed on that dragon's page.
Multiplayer[]
In multiplayer, there is an exeption for dragon feeding as when the player can't be online to feed their dragons 24/7. They could have connection problems and wouldn't be able to connect to the server. Therefore, instead of becoming feral and hostile, they will become neutral and will ignore their owner if he connects to the server. If the owner hits the dragon, he will turn hostile anyways. In order for the owner to get their neutral dragon to listen to them again, he simply needs to feed the poor, hungry dragon. The dragons will have a food meter showing for only the owner, and if this bar runs out, the player will be kicked off the dragon and the dragon will decend, becoming neutral.