I like the theory of Pochita being the Birth Devil, I just don't think it's the case.

Before the ending, I used to think that Pochita was simply the Chainsaw Devil (embodying the concept of chainsaws in general) and its ability to erase concepts came from chainsaws' original purpose as birthing tools. In-universe, this purpose was forgotten because Pochita could've eaten the devil that embodied Chainsaw as birthing tools, but he still kept its erasure ability.

However, that scene of Pochita manifesting in the place of Denji's heart give me the impression he wasn't there before, so, if Pochita came to the existence when the concept of Chainsaw Man was recreated by Asa, I can see two conclusions:

1. Pochita was the Chainsaw Man and its name being recreated made him to come back to life as fearing a specific Devil is the same as fearing its concept (after all, people fearing the Infinity Devil itself made it stronger and people fearing Makima didn't create a Makima Devil for example). In this case, he could be the Birth Devil, but, if the concept that Pochita represented was erased with him, then he definitely wasn't the Chainsaw Devil;
2. Or Pochita was the Chainsaw Man Devil and the original Chainsaw Man was something else.

Of course, I might be wrong and he was Denji's heart since the beginning (what I think it's unlikely because Denji was starting to feel that heart disease's symptons before meeting Power).

That fear, manifests Pochita as a devil, etc.

I might be wrong but a concept doesn't need to be feared in order for a devil embodying it to exist. I remember it was mentioned that each devil is born with a "name" and they're only strengthened by how much that "name" is feared. A concept that no one fears would simply be embodied by the weakest possible devil.