I'm trying to build background information for a tabletop game and one of the things I wanted to have be a factor was upkeep such as keeping creatures fed. I am intending for players to be able to purchase/tame creatures and ride into battle or fight alongside them. Upkeep seems an ideal way to limit how many/what they can take into battle.
Is there a method for coming up with a rough approximation of how much food various mythical creatures would need based on factors such as body mass and abilities? I'm looking to determine a relationship between calories and mass/abilities of the creature that I can use as an approximation which I can adjust for game balance or adjust the creature's abilities based on its upkeep.
Dragons, for example, would need something their bodies can process into fuel for their fire breath attacks (strictly non-magical in this setting.) I believe they can process most living matter into a fuel for their breath attack. There are plenty of questions on the fire breath example, so I'm not too worried about this specific point.
Another example, as mentioned in the title, is the griffin. Since it has a half-lion body, would going off the dietary needs of a real world lion be fitting? Or would its flight equipped body need more/less food due to the combination of extra muscle work for flight and a lighter body needed for flight?
For dragons, wyverns and other larger creatures, would going off estimates of caloric intake of similarly sized dinosaurs work?