Night howlers

Night howlers (also known as Midnicampum holicithias) are toxic flowers that can turn civilized mammals into savage beasts.

Bellwether and Doug used the flowers to concoct a serum to turn predator mammals aggressive, as the central part of a master-plan to take over Zootopia by turning the prey (the majority of the city's population) against the predators. There were multiple victims, including Mr. Manchas the black jaguar and Mr. Otterton the otter.

Description
The night howlers are small-medium flowers that have six violet petals and several yellow pistils and stamens. Night howlers are grown from root bulbs, which can easily be mistaken for onion bulbs.

Effects
Night howlers contain toxins that are capable of inflicting highly potent psychological effects on mammals. When consumed or injected, the flowers' toxin causes a chemical reaction in the brain that decreases activity in most parts except the cerebellum, which becomes hyper-stimulated, causing the victim to lose higher reasoning functions and lash out at anything that moves. The effect of this is that essentially, any mammal reverts to an aggressive, wild, violent state that is described as "savage".

The night howlers can even affect animals that do not have a naturally violent or ferocious disposition—as evidenced by their ability to turn non-predator animals, such as rabbits, aggressive. For example, Bonnie and Stu Hopps mentioned that Bonnie's brother, Terry, once ate one whole and went berserk, attacking Bonnie and "biting the dickens out of her".

As natural flowers, night howlers can outright kill small animals such as insects, which makes them an effective pesticide.

Usage
Stu grows the flowers around his crops to scare off bugs, but has his children and other fellow rabbits stay away from them to prevent exposure.

Bellwether orchestrated a plot to use night howler chemicals to turn predators in Zootopia feral, giving the illusion that they are biologically reverting back to their "primitive savage ways". One of her henchmen, a ram scientist named Doug, uses his chemist expertise to modify the flowers' mind-altering toxin into a concentrated pellet, which can be loaded into a dart gun for long-range injections. Doug uses his sniper aim to dart every predator he can, especially ones that could risk exposing the plot. Duke Weaselton was paid to steal bulb roots of the plant for Doug to create night howler darts.

After Bellwether was exposed and arrested, doctors created an antidote for the afflicted predator mammals, including Mr. Otterton, who was reunited with his wife.

Role in the film
The night howlers were mentioned early on as the last thing Mr. Otterton talked of before going savage. At the time, Judy and Nick assumed that the name referred to the wolves involved in the disappearances of the other animals around Zootopia. Judy also thwarted one of Duke Weaselton's thefts of night howler bulbs, although the owner of the store believed them to be onion bulbs at the time.

The flowers themselves first appeared at Bunnyburrow where Stu uses them to scare off bugs away from his crops. When she heard about the severe psychotropic effect they have on mammals, Judy realizes that she made an error—"night howlers" referred to the plant, not the wolves—and rushes back to Zootopia.

After reconciling with Nick, the duo get information from Duke, who was doing it for money. He states that he did it for Doug in an old subway car. Judy and Nick go to the subway car, which is filled with night howlers being used in experiments, and they watch Doug liquefying them into a serum. Doug then gets a phone call from his employer, who informs him of his next target, making Judy realize that Doug is responsible for making Mr. Otterton and Mr. Manchas go savage by shooting them with the night howler toxin.

At the natural history museum, Judy and Nick discover that Bellwether is the mastermind behind the night howler operation. After her henchrams knock them into a pit, Bellwether fires a dart at Nick, intending to make him go savage and kill Judy to conceal her role in the plot. The seemingly savage Nick appears to bite Judy, but it is all an act; the duo had replaced the night howler serum with blueberries from Judy's farm before Bellwether reclaimed them. Bellwether threatens to frame the two like she did with Mayor Lionheart, but Judy reveals that she has recorded the former's confession with the latter's carrot pen, just as Chief Bogo and his Zootopia Police Department forces arrive and arrest her.

At the end, an antidote for the night howlers was created, curing all afflicted predators, including Mr. Otterton.

Trivia

 * According to Judy, the night howlers are related to the variety and are a class C botanical.
 * The word "Midnicampum" is loosely derived from Latin meaning "between the fields", while "holicithias" has Greek connotations as "all shepherd", alluding to the plant's use as a field pesticide.
 * Alternatively, the plant's nomenclature parallels the colloquial name: "Midnicampum" is similar to "midnight", "holicithias" is close to the word "howl" when pronounced phonetically.
 * Victims of the night howlers cannot spread the savage disease onto others. This was shown when a bite from a savaged rabbit did not affect Bonnie when she was younger, nor did Manchas become savage by an afflicted Mr. Otterton, despite sustaining severe cuts and slashes from the encounter.
 * In an early draft of Zootopia, night howlers only caused animals to revert to their feral instincts instead of making them mindlessly violent. In this draft, Nick was successfully poisoned by Bellwether, but did not attack Judy since his instincts saw her as something to protect, due to their established friendship.
 * As a side effect of night howlers' exposure, the eyes of afflicted mammals will typically resemble those of their real-life counterparts. The most notable example of this is Mr. Manchas; in his natural state he has dull green eyes with round pupils, but while under the influence of the night howler sickness, his eyes are a much brighter green with slit pupils (like that of real-life cats).