That's not what I meant. What I meant is that for example venom is fairly "expensive" for the animals that use it to produce; whatever biological processes now are involved in secreting the stuff require energy. Sort of the same way as using them brains is rather energy-sapping for smart animals (as most no doubt know from experience, extended periods of serious intellectual stimulation, say exams, are very tiring). Ergo, the animals that use it do so rather frugally, and only for a reason. Venomous serpents for example only inject as much as they "think" is necessary to take down the victim, and if it doesn't drop will try again. I understand young serpents are actually often more dangerous than adults, as their control of the "dosage" isn't as good and they often just pump the entire venom sac into the target especially if frightened.

Others use different applications. Komodo dragons for example mix it with their saliva, and if they can't take their prey down initially will simply chase it until the rather diluted venom weakens it enough for the lizard to move in for the kill.

I don't know the details, but I'd imagine producing a full-strenght jolt is also pretty physically exhausting for electric eels and rays. The energy has to come from somewhere after all, and maintaining enough of it tends to support vital functions tends to be among the main concerns of all organisms.

Now, what the point is is that gathering, producing and storing suitable "fuel" for an organic flamethrower would not seem to be any less of a considerable undertaking - all the more so if some of the chemicals involved are of a sort that needs to be synthetized from raw materials. Thus, an organism is unlikely to invest the energy and effort required unless it has a good reason to.

And I really fail to see why what would sound like a cross between a crocodile and a big monitor lizard would have the slightest need for that sort of complicated chemical weapon. High-end carnivores in general get by right fine with just being fast, fierce, well armed with tooth and nail, and/or good at ambush/pouncing attacks.

Most of the reasonably easily produceable flammable chemicals I can think of don't make terribly good incendiary weapons either - assorted alcohols spring to mind, and those have pretty low burn temperatures. Petrochemicals are Right Out - the natural processes that produce them are in no way reproduceable inside a living creature, and their natural supply is very limited as far as an animal is concerned (natural seepage of crude oil occurs in certain regions, but that's about it). Moreover, I don't think most animals are too flammable either. Certainly not to a degree where breathing fire on them would be an effective hunting tactic, plus that has unacceptable amounts of possible side effects (like, say, setting vegetation aflame - I doubt these supposed "dragons" particularly enjoy being charbroiled by brushfires...). One would imagine the primary function of a burst of flame would be psychological - animals fear fire, after all. But what the heck a major carnivore would need it for is beyond me. Those guys tend to sit at the top of the food chain with few natural enemies, and are more likely than not to avoid direct physical conflict with other superpredators (since the risk of a crippling injury rather outweighs most other considerations). They're not going to need a weapon to drive away enemies, since they as such have none (except others of their own species, and that mainly around mating season) in the first place.

Complicated and effective chemical weaponry is AFAIK most common in the lower and middle regions of the food-chain pyramid. Smaller predators, which lack the sheer horsepower and natural weaponry of the big superpredators, often use chemical warfare to help in defense against bigger carnivores (think skunks, or venomous frogs), offense to take down prey or both. The big critters at the top of the pile simply don't have much need for such complexities, although some - like the Komodo dragons - use it as an auxiliary weapon anyway, but then that may just be an evolutionary remnant that's stuck around because it's useful enough to pay itself back.

In short, even ignoring the practical chemical and physiological problems involved in a "living flamethrower", there's no logical reason why a large carnivore would evolve such a complicated chemical weapon system. It simply wouldn't need one for anything.