Not sure if anyone has already brought this up, but I think unicorns could fly if they wanted to...? We've seen it multiple times in the show, after all.
But it's kind of weird that unicorns are able to do this, since pegasi are known for being the "flying ponies." Are they trying to make unicorns op?
Also, why don't unicorns fly more often? Do they just choose to walk around as a show of good faith to the pegasi?