Blood for the Baron!

I don't want to get an archenemy but I will also point that Unicron is NOT a Decepticon. I would rather had said that there is no 'con beginning with U.

Seemingly, the same thing happens with X, Y is for Yokuryu and, I know I'm saying two, but...

Z is for Zarak! Lord Zarak! One of the best Decepticon leaders! (equal to Shockwave, I can't decide who is the best 'Con leader between those two).

new theme then:

aliens in Star Trek

Andorians

We are the Borg. Your existance as you know it is over. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.

C for Cardassians

Dominion.

the Dominion's not really a race, more an organisation, but i'll let it slide...

Efrosian

Oh goody Ferengi.

Gorn

Halkans

I for Irish O'Brien is an alien to a Klingon etc