One of the many heads severed from the old body of Ai Coleman, this head in particular belonged to the Kaiman persona, and is the only one that wasn't destroyed, mostly because it was kept by the lizard head, dissected and put in a formalin jar in The Hospital and then stolen by Curse.
The anomaly with the last hexed head of Kaiman is mostly due to the treatment the head received in The Hospital, the spikes on the back were cut, the eyes lost the irises and the scales became more pale thanks to the formalin effect.
Back in Chapter 8, Kaiman was killed by Shin when the cleaner used his hammer to tear the lizard's head off, while Nikaido was overpowered by Noi, though in the last minute, she escaped from the fight, taking the beheaded body of her friend and the head to The Hospital.
Next day, when Kaiman regrew a new head (still hexed) and Vaux asked him if he was going to dispose of it or keep it in formalin, both ideas seemed gross and disturbing to him at first, but in the end he learned how to dissect it. Nothing came of Vaux's examination attempts in order to learn Kaiman's body's secrets, as the results were simple, it was a common, human, though hexed, head.
The same night, a blackout turned all the light in The Hospital off and the head disappeared without a trace, making Kaiman furious.
In later chapters, we discover this was done by Curse. By this time the "man inside the lizard head" was nothing more than a ghost living inside, due to the original cause of Kaiman's hexed head, Ebisu's magic, being weakened as she was on the verge of death. Curse was able to take the head from the lab and go unsuspected thanks to the power blackout, coming out and returning to the head, though the reason behind him returning instead of trying to kill Kaiman right then and there is still a mystery.
In Chapter 61, considering the simultaneous death of both Kaiman and Ebisu, Curse was released from his body and the spirit left the dissected head behind. It was recovered by Kasukabe and Shin and left in En's Mansion, in Nikaido's room to be more specific. It was meant to be some sort of souvenir to her, but also a trace of hope, knowing the real Kaiman was alive somewhere, seeing as how it was impossible for it to be the last of his heads, considering its origins and remembering Kaiman no longer had a hexed head.
The head takes a prominent place in the story once again in Chapter 118 as the almost certain possibility of being the source of "The Second Kaiman" existence. Nikaido theorizes the Kaiman in front of her was resurrected using the head, explaining why the new one has his spikes cut and why he does not remember anything since the last day of The Living Dead (Chapter 8).