I hadn't noticed before today, but my little guinea pig (about three weeks old) has a parasitic twin! Well, sort of... she has an extra toe on her foot. It's all loose and dangly and sits back on her foot like a dew claw, but it's full sized. There are no bones attaching it to her foot, just a little bit of skin, so it's really gross - it twists around, wobbles, bends the wrong way. It's going to be removed, just a little freezing and a quick snip, at least with no bones it is easy to take off, more like a skin tag than an actual digit.
Just thought I'd share. It was so weird, when I first saw it I thought she was supposed to have four and one was badly broken.