The only way to alter their spawn rates is to add some lines to your config file (game.ini), specifying the rates. You can give this a go- it's the lines from my game.ini designed to work on custom maps w/a dino spawn rate at 1.2% in the advanced settings. Killing some wild dinos in an existing game & waiting 2-3 in game days SHOULD start spawning more dragons. Just got a Dodo Wyvern today on Ragnarok :) https://pastebin.com/UDMrZc2K