Persanaly
I think Pinkie Pie would have a Moustash, like she had in season 3, episode 9 spike at your service

and I think A.J. would have a lumberjack beard

What about you, who do you think whould have FacialHair ?

I love how Celestia and her male counterpart have the same energy.

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Applejack would have a beard like Wendy's father, from Gravity Falls.

I can see Elusive sporting one every now in then depending on what’s fashionable.

Butterscotch I can see and looking like Tom Bombabild or Peter from that one early ep of Family guy

The one I definitely do not see having any facial hair is Blitz since he would think it messes with his aerodynamics

Applejack: Typical farmer beard, but she'd/he'd keep it trimmed so it was out of her/his way during work.

Fluttershy/Butterscotch: Pencil beard and mustache. The really thin ones.

Rainbow/Blitz: Goatee. Because Goatee's are universally "cool".

Rarity/Elusive: The style varies from short to long, braided to combed.

Pinkie Pie/Bubble Berry: Peach Fuzz beard, to keep it out of her/his way when baking.

Twilight/Dusk: Either a copy of Celestia/Solaris, braided, tied, trimmed, or a "wizards beard", ranging from big and full to "Doctor Strange" goatee.

Luna/Nocturn: Short and trimmed or peach fuzzy.

Celestia/Solaris: Long, glorious, "Wizards Beard".

Those are my thoughts on their facial hair anyway.