Zhen is a legendary Chinese cryptid known as Zhenniao (鴆鳥). It is a poisonous bird 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 that roughly translates to "quenching your thirst by drinking with a Zhen(niao)" (飲鴆止渴) references the cryptid, with the idiom being used to express "resorting to reckless/inconsiderate action for solving (an) imminent problem(s)."
"A bird in Chinese mythology said to have poison in its feathers, because it eats poisonous snakes. Dipping a feather into wine will turn it into a deadly poison, able to kill anyone with one drop."
—Shin Megami Tensei III: Nocturne profile
Zhen first appears in the overworld map, outside of the Shinjuku Medical Center and can be found outside of Yoyogi Park, Shibuya and Ginza as well. It is the first demon to learn War Cry as part of its skillset. Due to being a member of the Wilder race, it can only be recruited through chance during FULL Kagutsuchi or with the Jive Talk skill. Otherwise, it must be fused.
Shin Megami Tensei IMAGINE
"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.
Shin Megami Tensei: Strange Journey
"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.