There are other theories, which now seem to be more false than true, that say that the 'Monkey' or 'Portugas' parts of their names are nothing more than nicknames, given to them because of their personality.
For Luffy's side, it sort of fits. I mean, Luffy is pretty monkey-like.
Name doesnt matter. I myself have a different surename with my sister. No problem right?
luffy: ...to become the king of pirates!!
zoro: ...to become the world's greatest swordsman!!
nami: ...to draw the map of the world!!
ussop: ...to be a brave warrior of the sea!!
sanji: ...to find all blue!!
chopper: ...to become the best doctor in the world!!
robin: ...to find the true history!
franky: ...to become world's greatest shipwright!!
going merry: ..to Sail together with the strawhats Forever!
neilson (myself):....to watch houston rockets' game live and see yao ming in person...
i thought of something, i think he is luffy's brother and the reason they have different surnames,is like
ace is supposed to be a famous pirate right
if his name was monkey d. ace, than since the begginin of the anime, everyone would know luffy is ace's brother
it wouldn't be cool at all, so for everyone to be shocked when he first appeared he put different surnames in them
the reason they have different surnames in the anime, oda will tell us later, but i think that this is why.