You can unlink the scale values, and set the X scale to -100.
To ensure it's an exact horizontal mirror (especially if your scale is already animated, or your object is not full frame/offset from the composition center) you can precompose your layer and then preform the inverted X scale on the precomp.
Thanks for your response.
Please forgive me. I am new at After Effects
Can you elaborate on what you mean by 'preform the inverted x scale'
This is seeming to me that it's going to involve something 3D?
I want it to flip, but I also want animate kind of like a page turning. So it's very thin at one point. Right side up on one turn, and a mirror image on the next.
Doesn't involve any 3D. what Steve is referring to is that you may wish to pre-compose your existing items to keep possibly existing animation independent from the scale. E.g. you may have your object already moving and you may want to retain that move on the page. This would only be possible by applying the scale one level above and making the existing motion a pre-comp, as naturally all scaling will be based on the anchor point. If you were to scale a moving layer directly, you really only flip the layer content, not the motion... Just try it, it will become clear. and for a page turn, you would not need to animate scale at all - CC Page Turn is your friend or you simply rotate your pre-composed layer as a 3D layer.