Is it Girafarig?

I'm not ahead; I'm not behind
The middle's where I be.

Girafarig has two heads, one ahead and one behind - neither is both, so I'm assuming that the middle is logical (I don't know what I justsaid, either. :p).

And despite which way my head is turned
The former two I see.

While one head sees forward, the other looks the other way - ahead and behind.

I have a dark side to me,
e'en though daytime is my friend

Girafarig's 'dark side' is its second head, which is dark-colored and dark in the sense of being mischievous. But, Girafarig is a giraffe nonetheless, and giraffes are diurnal.

The actions I do now
might not hit 'till the end.

On the anime, Girafarig knew Future Sight, and in the game, Girafarig can learn said move through breeding.

Who am I? As it seems
that's up to you to guess

It may be difficult to determine which is Girafarig's true head...

But who I really am
I know better than the rest.

...But Girafarig knows, since its second 'head,' in truth, is missing a brain.