The song was part of an earlier story version, where there was an heir and a spare; Mewtwo being the heir, while Lucario was just the useless spare.
Since it ends with what is supposed to be the sound of Lucario crashing into Machoke's rapidash, it's implied that this is the song that originally would have been used in place of "For the First Time in Forever". Some elements of the song do get retained in "For the First Time In Forever," such as the song starting in F major and going up a half-step on each verse.

Lucario: Spare...? Am I really just the spare?

I'm not part of the town, not born to be king
Just somebody hopelessly in-between
he's the scholar, athlete, poet
I'm the screw up, don't I know it
But then who could ever compare?
Of course they're gonna think I'm just the spare
Well, I won't care

So I'm the extra button on a coat
In case another one comes loose
But if I have to be a button
Why can't I be a button that's of use? (Oops!)
I may lack style and I may lack grace
And once in a while I fall on my face
But this little button deserves a place in the sky
This button wants to fly

Wait, buttons can't fly, it doesn't make any sense!

So I'm a rusty horseshoe hanging up
Over somebody's old barn door
And I'll be hanging there forever
Just wishing the horse had one leg more
And maybe I can't be the perfect one
And maybe I am on the side of fun
But horseshoes need a chance to run somewhere
This horseshoe is more than just a spare

Someday I'll find my thing
A thing that's all my own
That thing that makes me part of something
Not just all alone
If only all this feeling I have in my heart
Could mean something to someone, how I'd love to play that part

So I'm just the second-born brother
Who most of the town ignores
Like a button, like a horseshoe
Like a boy who's bad at metaphors

Maybe I don't have a magic touch
And maybe I don't have a talent as such
Just this heart with much too much to share
So I'll never be the heir
But I'm more than just the spare *CRASH* (Sorry!)

song©disney
lucario©the pokemon company and nintendo

https://www.youtube.com/watch?v=66MQPWZ-n0g