Are there any birds whose legs fold the same way human legs do, in a knee, instead of an elbow which is what all the birds I know have?
The joint you are thinking of is not a knee, nor is it an elbow, instead it is an ankle which is bending the same way as us humans. You can see from the below diagram that the knee - the joint between the femur and tibia - is just further up the leg normally hidden by feathers.
Birds have a comparatively elongated metatarsus which gives the impression that the knee bends backwards more like an elbow, but it's just the ankle. (Image from here)
I also liked this diagram from a blogpost on a similar theme...
To follow up there appears to be no animals whose ankles bend the "wrong" way.
Though these are far from conclusive resources. I'd guess the way knee is defined (it means something to humans but not to nature (a bit like the species being a concept which has weak biological bounds)) has something to do with that