
XERO: About the only X I could find, Xero were an old NWOBHM British metal band.
Xero - Encyclopaedia Metallum: The Metal Archives (Bah! Not even a Wiki entry!)

Y&T: American Metal band who were most active around the eighties.
Y&T - Wikipedia, the free encyclopedia
I couldn't find anything for

other than ZZ Top, and I ain't calling THEM Heavy Metal!