I have been a black dragon around 10 times and a king dragon once, and using this strategy you can be a black dragon in one to two hours, if you don't get eaten.
First choose a server that has the least amount of players, because less players means less chance of you surviving. after that be a chipmunk and spit water at mushrooms then be an arctic hare and find a lemming and eat it, and you will skip penguin and be a seal. eat another lemming then make your way to a duck pond as a reindeer, and eat lily pads be a jellyfish then a turtle then be a snowy owl, then snow leopard, wolf, then be a vulture and eat aloe. be a bison and eat aloe then cactus. Be a komodo dragon, black widow then be a pterodactyl, but here things get tricky, the aloe has no affect making it harder to level up, you also need 5 million xp to level up, but after you have your 5 mill be a giant scorpion or a dino monster and then you can be a black dragon. but your chances of survival rely on you being good at 1v1 because chances are there will already be another black dragon on the server and most people like to eat you.