(And admittedly, FIM:TNG does take some liberties with the events of MLP:FIM, mostly in regards to Equestria Girls and all episodes after and including The Cutie Re-Mark. That will become more apparent when it comes to the character bios for Sunset Shimmer, Flash Sentry, Starlight Glimmer, and others.)
Now, I have done this for years now online, but I think it deserves attention here: whenever I discuss a couple, I always use an ampersand "&" between their names, whereas with platonic pairs/groups, I use the word "and" instead. And also, I think we all take it for granted that same-sex couples in _Friendship Is Magi_c headcanons can have kids. I mean, magic is a thing in this universe, and it's not ENTIRELY written off that sex-changes could happen with magic. Or just, you know, other options with the magic to allow two mares or two stallions to have children together. Anyway, moving on.
This is Octavio Slate Pie, Pinkie's brother. Yes, he is technically a character from Pony Life, but bear with me on this. It will make sense in a little bit. Because, you see, this interpretation of Octavio Pie appeared in the original show. You may also notice that his color scheme and cutie mark are extremely familiar…
Marble Pie always felt great discomfort in her own body. She hated to speak and so mostly relied on her twin Pinkamena to do the talking for them. And after Pinkie got her cutie mark and became so much more talkative, this became much more convenient. Marble let her mane grow out and hid her face with it. She stayed mostly in the farm, and with Maud off to earn her rocktorate and Pinkie off in Ponyville spreading her brand of cheer, Marble's oldest sister Limestone was left to run the farm with her more mellow parents tending to the gruntwork. (As Earth Ponies, they would retain their vigor for far longer than Unicorns or Pegasi.)
And then Marble met Big McIntosh. It was the greatest Hearth's Warming Eve gift that Marble could have ever received, though not in the way that an outsider could have reckoned. Marble herself thought that she loved him at first, and her heart ached that he could have his own special somepony when he came with Sugar Belle the next Hearth's Warming. But as she thought about it, and how she felt about herself, Marble realized that she didn't love Big McIntosh like that. Rather, she wanted to be like him. Marble Pie wanted to be a stallion.
It took more willpower than he could have thought to come out to his older twin. Pinkie was visiting for their birthday and exclaimed that she was so happy to have planned the surprise birthday party for herself and her baby twin sister. "…not your sister." It was the most he had said at once in a long time. He continued, "…your b-b-brother." Pinkie's confusion gave way to absolute joy. She had a baby twin brother now! She had to change her plans post-haste! This would also be an introduction of her brother to everypony!
As for his name, he always loved the name of his cellist cousin Octavia Melody. He wrote it down as the male version Octavio, and it just felt right. His middle name of Slate comes from the metamorphic rock.
In no time, Octavio was the scene of the party (which he interjected himself into the planning for). He spoke more in one minute than he had in several years. To many, it seemed like the backlog of words he'd held back as a mare and filly had come spilling out now that he was a stallion. He cut his mane and tail short, no longer hiding his face. Applying for transitional spells and potions, his features became more pronounced, his voice deepening. Though he wouldn't talk nearly as much as he had at his birthday/coming-out party, Octavio became a much more sociable and open pony than he had ever been as Marble. He finally felt like he fit in his own body. He was finally happy.
For a short period, Octavio took up the same sort of role as party-planner as his twin, to the point that many would have called his parties Pinkie Pie Parties by mistake. This led to several misunderstandings when Pinkie received rave reviews for parties she definitely didn't remember planning. They had all the hallmarks of her parties, though, so only one pony could have put them through: her brother. She confronted him, fearful that he was trying to outdo her, outshine her. Octavio insisted he would never; she was the first one who he'd confessed to of feeling like a stallion. After more discussion, Octavio decided to take a different path with his party plans. He wouldn't use Pinkie's ideas, but his own.
Having opened up finally, Octavio Pie is bursting with self-confidence, to the point of smugness. While this means that he will not hesitate to do something that is firmly in his wheelhouse, he can unfortunately put others off. When it comes to one specific situation, though, he can soften his behavior and project profound empathy: supporting foals struggling with dysphoria. He hates to imagine foals being in the same position he'd been in for years: hating his body, hating his voice, feeling like an alien in his own skin and bones. With his own brand of parties and penchant for providing pet rocks, he yearns to help these foals enter their new identities with the happiest moments yet in their lives.
Octavio no longer lives at the Pie Family Farm, but travels Equestria with his wife planning parties. He specializes in coming-out parties. Among the foals he has thrown such parties for were Piñata Apple and Pumpkin Sweet. At a certain point he got a tattoo on his right foreleg representing his "bolt from the blue" revelation of being a stallion, and it serves as more of a symbol of an Octavio-brand coming-out party than his own cutie mark. He also may or may not have a touch of "Pinkie being Pinkie" that was awakened after his own coming-out, but anypony who knows will neither confirm nor deny.
(also, I know that octavio's cutie mark in pony life is different from marble's and not a tattoo, but everypony's cutie mark is different in pony life, so if you take issue, kindly shut up)
(also also, I bounced between octavio having a husband or a wife, but I settled on a wife because hetero relationships between cis and trans people is valid as well)
(also also also, I hope that I am decent and respectful towards the experience that some trans folk enjoy when their euphoria bursts through)