thats right if you make fun of Derpy you will have to deal with her little filly and man can she put up a fight. *gets stabbed by Ghirahim for the 120th time* my goodness do you have stab fantasies or something!? Ghirahim >.> maybe.
It is a fan theory, read: It might be true, but we have no way to tell, however, given how much they are canonifying Derpy(in the last episode, she was wearing a MUFFIN saddlebag...) we might get a confirmation of this at some point.
I personally hope Dinky isn't Derpy's daugther, Derpy just isn't fit for a mother, however, she would be a great older sister.
"Derpy just isn't fit for a mother" Typical fanon is that Derpy is completely incompetent at every single thing she does EXCEPT raising her kid(s). It's a humorous contrast, and a heartwarming way to bring "power of love" into play. She may have destroyed town hall without even noticing, and she may put all the mail in the wrong mailboxes, but when it comes to her little muffin(s) she will not let ANYTHING happen to her/them: [link][link][link]
Yeah, but it isn't only the "raise the foal" aspect in there, we also have the "make the foal" there, look at Derpy, do you honestly see her dating someone?(sure, I saw pently fanart of her with Dr. Whooves in the past, but those also say he is a Time Lord, surelly not your everyday stallion and even surelly-er not Dinky's biological father) my point is, "mother Derpy" imply a very, very ugly backstory, I honestly hope she isn't Dinky's mother because I like Derpy too much to even think something like that is true.
And sure, you could do a "maybe Dinky is adopted" argument, but the whole reason fandom started to say they are related is because they look a lot like each other, changing this bit of fanon now is a little too far-fetched to me.