Rainbow Penguin - Where to farm?

with 80 stamina on penguin long beach I got 0 rainbow penguins. What are the actual drop rates here? On penguin long beach I have met 2 rainbow penguins in total and both time it dropped. Is it 100% chance to drop on this island if you encounter it and what is the encounter rate then? 

So the question is: is it better to farm rainbow penguin (not the pirate one) on penguin beach or penguin long beach?

You are saying rainbow penguin is a hidden boss in long beach? Didn't even know that but if you do encounter any hidden bosses then its 100% drop. Going with previous chance to get hidden bosses its probably 5% to encounter him.
With the half stamina this week for even isle i would farm him on regular penguin beach cause you will always fight him there thus having a chance it drops.

Ye, I have played 10 times long beach and he appeared once on stage 1 and 1 time on stage before boss. The normal penguin beach I have played total over 40 times and still no drop. So I am thinking maybe its better to do long beach to have an opportunity to get both rainbow penguin if tha chance for the normal rainbow penguin is the same in long and not long.

Try the Penguin Island today with half Stamina. ^^