The Da Peng Niao - where "Da" (大) means "big" or "great", and “Niao" (鸟) means "bird" - is the largest bird in Chinese mythology. It was first recorded in the Taoist classic, Zhuang Zi (庄子 zhuāng zi), an ancient text from the late Warring States period (战国时期 zhàn guó shí qí) (476–221 BCE), where it states the Da Peng Niao can transform into a massive sea creature named Kun (鲲 kūn) - and vice versa.
"In the dark1 northern sea there is a fish, its name is Kun. The Kun is huge, I have no idea how many thousand leagues2 [it measures]; It transforms into a bird whose name is Peng. Its wingspan, I have no idea how many thousand leagues [it measures]; When it lifts up and flies off, its wings cover the sky like clouds. It is a bird, it uses the turbulent waters to migrate to the dark southern sea. [For] the Tian Chi (天池 tiān chí)3 is located in the southern sea."
1 the sea is described as "dark", meaning "deep".
2 a Li (里 lǐ) is a measurement of length that has been used in China for thousands of years. However, the exact length is unknown and is believed to have changed multiple times over the years - which is why I translate it to "league". The modern day definition is 500m.
3 Tian Chi is often referred to in English, literally, as the the Pond/Lake of Heaven.