Usually, those kind of legendary ponies aren't the hidden clandestine kind. I guess those Twilight Sparkle devotees must've cast a temporary alicorn ritual spell onto their chosen one to ensure no pony in the open can discover their true power and take their powerful magic away!

WELL, I guess we all saw how THAT turned out! Now that her alicorn magic's an open secret to us, she's practically like a delicious peachy plum for the taking!