Penguins are not part of the Flying Castle ecology.
That said, the castle moat seems to have plenty of tilapia, despite the depredations of the resident plesiosaurs. Since they spend a lot of time in what I call the grotto, I don’t care to speculate what else may be a part of their diet.
After all, what happens in the Lower Levels, tends to stay in the Lower Levels.
Fluffy doesn’t give a Phhhhpht! about ecology.
Unless, of course it finds its way into the moat and ends up in the grotto. And then the denizens of the moat may have more than cronchy fishies to play with.
;o]