There is no difference between them. A muscular guy is correct.

overlapping meanings (some the same, but some very small differences) 1st one means a guy who has intentionally built up his body's muscle mass; the guy is very interested in muscles (but currently he may or may not have achieved his goal) 2nd one means a guy who has already achieved his goal (at least to a certain extent or has surpassed earlier goal) 3rd one means the same as the 2nd one (except 3rd one is formal English whereas 2nd one is suitable for informal communication such as being humorous or joking) - may not always be intentionally (for example, some men in physical labour jobs become muscular but it's not always a goal that they have) Cheers.

A muscular guy is correct

Hi I'm looking for a partner. It would be helpful for me Because help me to improve my English language . Can I speak with you or chatting with you?

a muscle guy - incorrect a muscled guy - incorrect a muscular guy - correct a muscley guy - correct