I designed a model in Blender, using an X and a Y mirror. When I tried to rig it, all the legs move with the one bone of the mirrored part of the model.
Here is what it looks like before:
And then when I move the one bone (the one that is the source of the mirror to the other three):
Moving any of the other legs does nothing to the model.
Why is this happening and how can I fix it? Is it because of the mirror modifier? I already checked the Face Orientation, and it was good.
Welcome to BA 
Yeah, you gotta apply the mirror modifier first, then it should work
Yes! It woks! Thank you so much.
One question: After I use this method, I cannot edit the object using the mirror anymore, as it is now a single mesh. If I wanted to change my model later using a mirror, how should I go about that? Thanks again.
It’s a bit tricky- you went to do as much of your modeling as possible before doing rigging/adding an armature. Applying the mirror modifier is a destructive operation, it permanently changes your mesh. You want your mesh to be as close to finished as possible before rigging, since you’ll have to re-rig when you make changes to the model
OK, thanks!