Potterheads and moviegoers know the Heir of Slytherin is none other than Tom Marvolo Riddle aka Lord Voldemort, but a new theory suggests that Dumbledore himself is another Slytherin descendant!
Alright now, take it easy and let's analyze what's being said.
According to Reddit user Obversa, a long and detailed theory explains exactly how Dumbledore and Slytherin could be related and it goes beyond Dumbledore understanding Parseltongue from "learning." This alone contradicts a factoid released on Pottermore which states that Parseltongue isn't learned, so much as understood by those who've inherited the gene.
Observa theorizes that Dumbledore is the son of Kendra Dumbledore, and as noted by 'Harry in Harry Potter and the Deathly Hallows' (Chapter 11), "Despite the high-necked silk gown she wore, Harry thought of Native Americans as he studied her dark eyes, high cheekbones and straight nose." The only other significant mention of Native Americans is the North American wizarding school of Ilvermorny- a school that was founded by a descendant of Slytherin, whose daughter went on to marry a Muggle from the Pocomtuc tribe.
Next, Observa takes the passage from Pottermore's information post
After some long backtracking through history and some mention of Kendra Dumbledore in the books, Observa notes it is unlikely Dumbledore learned the language so much as he innately understood it and could not speak it, similarly to the founder of Ilvermorny and his possible ancestor Isolt Sayre descendant of Slytherin.
Given what we do know from the series, of Dumbledore himself and his history, this theory holds true for what's brought up and even runs in accordance with some newer lore which was published years after the final book.
Now, I've heard some crazy Harry Potter theories that I don't quite agree with and even try to debunk, but I'm actually quite a fan of this theory as it introduces a new depth and aspect to Dumbledore. Granted, there are probably more secrets Rowling has yet to reveal of him from the movies and in her Pottermore published lore, but perhaps this one is right and Dumbledore is of House Slytherin!