>>29460408>Planned out my team all nice, with the only type overlap being Rowlet and Minior, though I figured they would be distinct enough for it to not matter too much, especially in an in-game run>This happens>Now I have no type overlap at allThank you John GameFreak.
Also, as for why it becomes ghost, consider how much owls are associated with ghosts in folklore and such (barn owls are also known as ghost owls), and as for why it loses its flying type, I just imagine that its long legs make it much more adept at running and leaping than flying.
And my personal headcanon for the ghost immunity will be that it's simply so fast that it's able to anticipate all basic head-on attacks, such as tackle. Note how its description on the website says it's usual cool and collected, but gets flustered when caught off guard. That would be a good justification as to why it's now weak to dark and ghost type moves.