This is why I'm a AlphysKnighter:

It's stated in Undertale that Alphy's believes in alterate universes, and it often times very invested in her fantasy stories, and many people are closed minded to Alphy's being the knight is the fact that she is shy and scared most of the time, but it's shown in a hotland aborted genocide route from Undertale that Alphy's can lead a nation of monsters during a genocide. My personal theory was that someone (Maybe carol) offered Alphy's a deal to be the knight, due to her connection to the studients (Aka Kris, Susie, the people from the prophecy) and knowing that Alphy's often times falls into escapism, the "person" offered her the position as the knight
Where she can be the powerful one for once
Where she can tear through everyone in her way
Where she doesn't have to be scared
I feel like it fits in very nicely
And would be a decent twist with clues when you replay it
Because people forget that the average player of Deltarune won't know about the Forgotten man
Gaster
They won't know Dess is in the code
So it's a twist that a average player can understand
As for the antlers:
It could just be a darkworld reflection of her goal
Another possibility is that the Knight's blackshard armor was given by carol
(Hence why BIG SHOT as the dummy motif)
(The body was created by The Mad Dummy)