Only Child is an original song featured in Original Song, the sixteenth episode of Season Two. It is written and performed by Rachel.
This song is her second attempt at proving her songwriting abilities to Finn, though she is still unsuccessful. It was inspired by her two dads having her as their only child. Finn says it is an improvement from My Headband and that Rachel is avoiding dealing with difficult (and more relatable) emotions. Rachel is initially argumentative but later takes his advice, writing Get It Right.
You've got just one egg
You're not going to make an omelet
Flying just one way
You're not going to make a jet set
Sister, brother, wherefore are thou?
No need for bunk beds in my bedroom, now
Oh, damn you, dads
Why did you settle for only me?
I'm an only child,
More than enough for them, but not enough for me
An only child
The only Berry on the tree
Never learned to share my Cher CD's
Don't got no other bit, not even a half-breed
Every tune's a solo in my home and my heart!
Only child, only child, only child
The only Berry on my family tree
The second verse was not released until after the episode.
The song was written by show composer James S. Levine and co-creator Ian Brennan. 
This, along with My Headband and several other short original songs and parodies were all released for free on the iPhone app, Tap Tap Glee.