can someone legally give a baby a last name thats not the mothers or the fathers?

Yes. It is legal to give your child any name you want, but you obviously have to be the legal parent.

Yes me and my babies daddy are not married and he was not on the birth certificate and I gave my kids his last name. No questions asked. SO yes you would be able to give your child an last name you wanted and no one would even say a thing.

Yes you can put any name you want on the birth certificate. But I ask why you would want to confuse the child in the future.