A legendary Chinese cryptid known as Zhenniao (鴆鳥), they are poisonous birds said to have existed in the mountainous regions of southern China. It gained its notoriety of being poisonous from eating venomous viper heads. A famous Chinese idiom "quenching by drinking Zhen" (飲鴆止渴) references the cryptid to express "resorting to reckless/inconsiderate action for solving imminent problem".
"A venomous bird appearing in Chinese legends. Its body is as large as an eagle, and its beak is the color of polished bronze, or reddish-black. Its feathers are a brilliant mix of purple and green. These feathers are infused with a venom so deadly that plants wither and die wherever it flies. It is said that in ancient China, the wings of the Zhen were cut off and steeped in rice wine to make a poisonous brew that was used in assassinations. Since there were no known venomous birds, the Zhen was long held to be a mythical creature. However, the recent discovery of a bird with venomous wings in New Guinea has raised the possibility that the Zhen may actually have existed in the past."
—Shin Megami Tensei IMAGINE Compendium
Zhen can be contracted at all times of day on Shinagawa field. It is one of the demons that can be mounted.
"A highly poisonous bird of Chinese lore. It likes to eat snakes with its copper-colored beak. It carries an incredibly potent venom; if it travels over a field, all the crops below will wither, and if one of its feathers falls into one's drink, a single sip can kill a man."
—Shin Megami Tensei: Strange Journey Compendium
In an EX Mission, the main character is asked by Dionysus to fetch feathers from a Zhen to use in a poisonous wine.