I had a child in 2016 but my girlfriend is claiming the child on her taxes. Do I even mention the child since she has already been claimed?

[ToddL]

No, you would not include any information about that child on your tax return. 

A child can only be claimed as a dependent on one tax return, If the mother has already claimed the child on her return, you would have to file as Single (w/o dependents, unless you have another child you can claim).

[fanfare]

This all depends on who the child is living with. Is it you, or her?