I speculated that Nagini and Fawkes (discounting the revelation in Fantastic Beasts that Nagini is a Maledictus) were the respective daemons of Voldemort and Dumbledore, given the nature of the humans’ connections to their animals. (I speculated that this was possible because Salazar Slytherin knew how to cross between the worlds in the HDM multiverse, as Dumbledore also did; and ever since then, Slytherin’s descendants have had external daemons.) Something against this theory is that, if Nagini and Fawkes were daemons, then Harry would have been spared the trouble of killing Voldemort when Neville killed Nagini, as Voldemort would have died when his daemon was killed—or had he made himself impossible to kill in that way? Less refutable is that, as Dumbledore did not use Dark magic to attempt immortality, Fawkes would have disappeared the instant Dumbledore died, rather than taking flight and leaving the school forever after Dumbledore’s death. But this theory could still be true as phoenixes are immortal—they are reborn from the ashes every time they “die” in flames—so if a daemon took the form of a phoenix, it might go back into the wild rather than dying permanently.