Nature Structures

As for me, there's only a point in building phoenixes and dragons in the Campaigns. It's easier to build a portal and hire locusts or honeybees there.

The whole point is in the damage, generally speaking. For Water Dragons, it's fixed (magical, of course) and can only be reduced by resistances, and it's generally small. It seems to deal 75 damage with 1 Water Dragon lightning bolt. Or something like that. A phoenix, hitting at the first level, can deal, say, 200 damage.
And so on.

I'm going with tigers because, as a rule, at the beginning of the game, there's simply nowhere to get 20 wood for elves.