What is this symbol?

It is an ampersand.

source: http://engage.riggspartners.com/r-blog/2offerings/amper-what-the-origin-of-a-symbol

... so Trump is going all mediaeval on someone?

Italy, it would seem.

And A -Z.

"When ‘&’ was the 27th letter of alphabet i.e. X, Y, Z and ‘&’, the school children used to recite the alphabets as X, Y, Z and per se and. “Per se” means by itself, essentially it becomes X, Y, Z and by itself and. So the word “ampersand” is formed by the wrong pronunciation of “and-per-se-and”. Such words are called mondegreen."

That's not strictly true. A "mondegreen" is a misheard line of poetry, or a misheard song lyric. This is from the Wikipedia article:

American writer Sylvia Wright coined the term in 1954, writing about how as a girl she had misheard the lyric "...and laid him on the green" in a Scottish ballad as "...and Lady Mondegreen"

At the very least describing ampersand as a mondegreen is anachronistic.