Itachi's Crow

This crow had Shisui Uchiha's left eye implanted into it.

Background
Shisui wanted to use his Mangekyō Sharingan's Kotoamatsukami to stop the Uchiha clan from carrying out their planned coup d'état against Konohagakure. Before he could do so, his right Mangekyō was taken by Danzō Shimura. Fearful that Danzō would also take his left eye and use it for ill purposes, Shisui removed his remaining eye and entrusted it to his best friend, Itachi Uchiha, asking that Itachi use it to protect Konoha. The eye was implanted within a crow for safe-keeping.

In Itachi Shinden: Book of Dark Night, Shisui called on a passing crow immediately after he removed his eye. He merged his eye with the crow's, which the crow did not resist. Once the procedure was finished, the crow nodded and flew off into the night. From that point onwards, the crow followed Itachi from high overhead, such as when he left Konoha after the Uchiha Clan Downfall.

Itachi Pursuit Mission
Itachi sends a crow clone to meet with Naruto Uzumaki. The clone questions Naruto about Itachi's younger brother, Sasuke, to which Naruto responds that he is more of a brother to Sasuke than Itachi is. When Naruto tries to attack, Itachi's clone uses a genjutsu on him, surrounding Naruto with a flock of crows. The clone asks what Naruto will do if Sasuke ever tries to attack Konoha. Naruto vows to protect the village without killing Sasuke. Itachi's clone smiles and has one of the crows fly down Naruto's throat, giving him some of it's power and telling him that he hopes it will never be needed. Naruto asks for an explanation, but Itachi's clone instead disperses, saying he has other business.

The next time he sees Naruto, Itachi explains what the crow's purpose was: Itachi had programmed Shisui's Mangekyō Sharingan to use Kotoamatsukami on anyone in possession of Itachi's Mangekyō Sharingan, compelling them to protect Konoha. Itachi's expectation was that this would be Sasuke, who would at some point need to replace his own eyes with Itachi's in order to escape the blindness that comes from using Mangekyō. If, after he did that, Sasuke attacked Konoha, Naruto would inevitably confront him, making him both the ideal container and deliverer of the crow and its eye.

Fourth Shinobi World War: Confrontation
During the Fourth Shinobi World War, Kabuto Yakushi revives Itachi using the Impure World Reincarnation. Although he doesn't want to, Itachi is forced by Kabuto to fight Naruto and Killer B, and in the course of the battle he uses his Mangekyō Sharingan. This activates the crow within Naruto, causing it to emerge from his mouth and use Kotoamatsukami on Itachi. Kotoamatsukami's instructions to protect Konoha override Kabuto's control, allowing Itachi to team up with Naruto and B to defeat Nagato.

Because the transplanted eye can use Kotoamatsukami only once every ten years, its premature use on Itachi has ruined his original plan. As such, Itachi destroys the crow with Amaterasu so that Shisui's eye won't fall into the wrong hands.