Board Thread:General Discussion/@comment-4504826-20160412211101/@comment-27701762-20160413013254

It's not being a monster per se. The bullets that we encounter are magic, which certainly composes the bodies of monsters, but is not something that is strictly speaking restricted to monsters (since humans, at least at one point, had access to magic).

Though the overall point still stands of how Flowey has access to magic. I suppose the answer could have something to do with the lasting monster essence which gives him Asriel's "personality" (for lack of a better term). Asriel's dust being infused in the flower gives Flowey magical powers that would have otherwise been lacking.

Or maybe plants simply have access to magical powers inherently, but lack the cognitive faculties necessary to make use of them, which are suddenly granted through the injection of determination.