It might be unknown, but I’d say it’s Rarity. She seems the type to have one of those fancy, unreadable signatures.
 
Rainbow is a tomboy, so I just can’t see her ending off a message with “Love.” And I don’t think Dash has ever spoke of herself in the first person outside of introducing herself in MMDW.
 
I know Dash can be in love with herself and not very humble, but I don’t feel like she’d acknowledge how in love she in with herself. The wording itself implies someone else wrote it.
 
It seems like the only reason Dash is there is because someone took a dig at her and she’s reacting to it. It can certainly be a love triangle situation, but I just can’t see that card actually being written and sent by someone like Rainbow.

removed the shipping tag, it’s not shipping, the artist confirmed it on DA
 
“Tragically, Fluttershy would never find out who sent the card because the signature was completely illegible.”
 
i mean it’s just VanillaGhosties joking around, but this still doesn’t warrant the shipping tag

@Claw  
Cause the card is written everywhere except the person who sent it, and dash looks more embarrased and annoyed about the cards context, saying that she loves herself

@Pikachu55  
And you think its not Sashie BECAUSE……?

Is this flutterdash though? Cause the card isn’t from Rainbow Dash

@Claw  
“It’s not like I like myself or anything like that.”

baka