Shimmering Spectacle 
sTs trinity
That is anatomically impossible, and considering that rhinos were thought to be unicorns, not even they can retract their horns, cause the horn would be in the way of their nose and mouth, besides a unicorn's horn is just bone, and bone itself is not flexible, it's the joints in between two or more, except in the head we're every joint (except the haw) is fixed, so I say again this is just impossible.