In the end of the Fantastic Beasts second movie Griundewald said that a fenix only appears to Dumbldores. And we can see that fenix appears before Credence. Plus Grindewald said himself that Albus Dumbldore is Credence brother. Plus we all know that Albus had a sister who could not use magic so she grew a shadow inside her. Three muggles raped her. It is possible that Credence can be her son, because he has a shadow too because he can not use magic.
Another theory is that a shadow escaped from Albus sister and moved to Credence. In this way he has like a soul from Dumbldore's so Fenix appeared to him.
Last theory is that fenix has nothing to do with Dumbldores. Fenix just appeared. Plius in fantastic beasts 2, Credence found a hurt bird which turned out to be that fenix. It is possible that Credence is Grindewald's son.
What do you think? Let me know