So, TL:DR. How do I make a custom creature available for trade via elven caravans, preferably without them appearing in the wild? If they have to appear as wild creatures, how can they be made very rare? What creature/entity tokens would be necessary?
The long version. If there are other ways to achieve my goals here, please feel free to suggest.
I'm tired of not being able to tame megabeasts properly in forts anymore. It occurred to me that I might be able to get around that by splitting the relevant megabeasts into two variants in the raw files; a "wild" variant, which would be the original megabeast with a different name, and a non-wild variant that is available via domestic animal lists, or preferably exotic animals (via elven caravan trade).
Take Hydras as an example. The normal Hydra megabeast would remain; I would just append "wild" to its name in the world. I would create a new Hydra variant, without the "wild", and this creature...well that's the question here.
It goes without saying that, if I make this creature available as a tamed creature that either the dwarves or the elves have access to, that its price would be absolutely enormous, in order to make its acquisition difficult. Thats easy enough. Now, I know that once you bought a breeding pair, it would be easy to trade them for entire caravans-worth of stuff, but I'm more than happy to impose rules on my own behavior as a player. Alternatively I can simply make it impossible for them to breed or have young.
So, in the case of making this example non-megabeast-token-Hydra available to elves as something that they would trade in a caravan, what tokens am I going to need on the creature, and even on elves as an entity (entity_default changes)?
I guess the rub here is that I'd like to accomplish that, but never see these guys in the wild (I know they'd appear in towns, but oh well), but I know that may not even be possible to do in the first place.
So, thoughts? Suggestions? Ideas? If there is some other way to get an analogue to megabeasts available for taming, without the bugs associated with megabeasts, I'd love to hear your idea on how to accomplish it.