So X-Blast's part of the theory is: So there are 3 most commonly accepted theories that I wil go over and you can choose which one you like the best. So Number 1: Is Dipper actually Dipper's real name? I don't think so because of Wendy's off hand comment in the episode Double Dipper "Oh so that's how you got your nickname!" Think what you want, but that's the most unlikely.
Number 2: Roderick is Dipper's real name. This is because Quentin Trembley calls Dipper Roderick in Irrational Treasure. But I don't think this is true because Quentin Trembley was the silliest man alive so that was just probably just him being silly.
Number 3: This is the most interesting theory I found. Is Dipper's real name Gable? Now this may seem weird, but there may be some evidence behind it. Now it's proven that the Pines family likes to name there kids similarly (Stanley and Stanford.) And Gable is a real name and its meaning has something to do with triangles. (Cuz I mean, we all ship BillDip ~Swift) This is a theory I found from the Internet.

Now to my (Swift) part of the theory:
So with the name theories I believe it might be Roderick. Possibly Mabel mentioned it to Quentin, cuz durp, he's a president and it's kinda obvious that Dipper doesn't like his real name.

Now on our IG post, the account irldopper brought this idea to the table:
They believe Dipper's name starts with M. Since Alex/Ariel, Stanley/Stanford, Mabel and .....? They all start with the same letters.