Well, I did some hacking and checking with the Z1 ROM and found this out (with Octoroks, anyway; I'm guessing other enemies follow suit):

If the walk flags are like this:
XX
X_
then the enemy can walk onto it from the bottom.

If the walk flags are like this:
XX
_X
then the enemy can walk onto if from the left.

If the walk flags are like this:
X_
XX
then the enemy can walk on to it from the top.

If the walk flags are like this:
_X
XX
then the enemy can not walk on to it at all.

Still have to check some other configurations, though.