Here’s my idea:
 
Flash Magnus had a relationship with his comrade Nimbus Dash sometime before he was sealed away with the Pony of Shadows.
 
In Pegasi families, it’s actually a tradition to name their child after one of your ancestors, because they believe it brings good fortune.
 
So after many generations, Flash’s parents named him after Magnus, however the identity of their other ancestor was lost to time… on their side of the family.
 
The other side of the family with knowledge of Nimbus named their daughter Rainbow Dash.
 
As such, both RD and Flash are actually distant cousins.
 

 
Thoughts?

I have a headcanon for that.

Would it be possible for Flash and Rainbow to share Magnus as a common ancestor countless generations back?

“similar mane”

Most likely he is Flash Sentry’s ancestor .

@Phantom Rider  
It’s possible, I mean IDW confirmed that Shadowlock was Stygian’s descendant and Cattail was stated to be Meadowbrook’s as well…

I guess he could be an ancestor if he did any babymaking before the whole Pony of Shadows incident.

@Background Pony #A8C5  
@Background Pony #A8C5
 
Stop throwing a temper tantrum just because you hate the guy. They wouldn’t have designed them exactly the same as he didn’t want them to be related in some way.

@Wing8492  
he’s not Ancestrys. they are not related

@Siegfried  
flash sentry is not descendent, rainbow dash is his spiritual inheritor. stop interested in those names and does not mean that they are the same.

@SeraphimDawn  
Revision of your headcanon: Flash Sentry is decendent, RD is spiritual inheritor, Flash Magnus is captain america
 
@The Wandering Pikachu  
Incarnation into your decendent is called transmigration.

@SeraphimDawn  
That doesn’t exactly work considering Magnus would have to die for that to happen.

@darknessRising24  
Headcanon: Dash is the reincarnation. Flash is the blood descendant.

@The Wandering Pikachu  
No dude, I get it. I understand that you were saying it was impossible for him to be a reincarnation at this point, I was actually defending the descendant angle by saying there wasn’t much of a confirmation that they didn’t have any children.

@darknessRising24  
facepalms If you’re talking about the whole group of ponies in the finale, at least one of them did, if you didn’t see “A Health of Information” (an episode I was glad to have, much like the episode before it, given last season’s lack of Zecora speaking in her two brief cameos in CMC episodes). No offense meant, but you obviously failed to understand I was stating that Flash Sentry clearly CAN’T be Flash Magnus reincarnated, for obvious reasons from the finale that was leaked already (much like last season, when it was leaked early, even if I would’ve waited under normal circumstances and did until a couple days ago). But I know people can still hold onto their headcanons, even if something in canon can later go against it.
 
I’m not saying reincarnation isn’t possible, but even if someone gets reincarnated, it’s usually not in someone else that’s related to the person they’re reincarnated as (tl;dr a reincarnated person usually doesn’t go in a descendant of theirs, if they already exist); reincarnation tends to disregard the physical act of breeding and a spirit can’t really be reincarnated within the family, for obvious reasons of being born already with a soul. :P And again, I said as for myself: I have my doubts that Flash Sentry is distantly related to Flash Magnus… but hey, even if some of them didn’t before the events revealed in the finale, they can now. ;)
 
Of course, I could’ve not said any of this, but I thought I might jump on the bandwagon on this image possibly before other people do, despite that it’s still a couple weeks until the public airing of this season’s finale. :P And I figured I might forget before then… but probably shouldn’t have anyway, since again, you misunderstood what I meant. No offense.

@The Wandering Pikachu  
Well it’s never stated if they had children or not, so….

I wouldn’t be surprised if they made Flash Magnus into some sort of rock star legend in the EqG universe.

Let’s go for Ancestry and and family tradition of joining the military. Some making a carrer other just doing their service.

@darknessRising24  
Now I imagine Flash Sentry being a fan of war metal bands.

@darknessRising24  
That’s my headcanon as well.

Ancestry plus Flash Magnus maybe it’s named after Ultra Magnus from Transformers.

@darknessRising24  

 
Great Idea man. (Not sarcasm)

Ancestry, my personal headcanon is that his parents named him after Magnus and Flash joined the royal guard in order to live up to his ancestors name.
 
Makes me wonder if Magnus was a war hero in the EQG world…