Wow, aliens is a hard one.
G is for General Zod.
h is for humanoids.
I is for Ironhide.
J is for Jazz
K is for Kal-El
L is for Laserman from Rockman exe
M is for Megatron
N is for nanobots.
O is for Optimus Prime.
P is for the purple people eater.
Q is for Quadrainian.
R is for rizegreymon.
S is for Stitch! 
little chef
T is for Toadborg
U is for Utwig.
V is for vectross
W is for Wookiee.
X is for x-saber
Y is for Yato.