WARNING! THIS POST CONTAINS SPOILERS FOR THE ADVENTURE TIME ISLAND MINISERIES. Proceed with caution if you haven’t seen the miniseries yet!
I just realized how Joshua and Margaret were able to name Finn his real name.
On the adventure time wiki, it states several times that Margaret and Joshua named Finn “Finn” because they thought he was a mud-fish at first, but as we all know, things change over time (Martin was originally going to be a hero, and Adventure Time was created without the intention of making it be a post apocalyptic setting). SO! I think I caught something in ISLANDS that the crew put in so Finn would still be “Finn”