Thought this was an Ameraucana

[weezyfno]

Can we get a profile shot of the OP's bird standing naturally? I'm thinking some type of game hen, but it's impossible to see body shape from this pic.

 

[junebuggena]

Looks like a game hen.

 

[weezyfno]

Looks like a game hen.

Are game hens usually aggressive?

 

[Rhodebar Lover]

Looks like a game hen.

Are game hens usually aggressive?

Yes, they were originally developed for cock fighting so she may seriously injure your Barred Plymouth Rocks. I have heard that some game hens can only be raised in pairs or trios because they have such a strong fighting instinct.