There's goatlike 'backwards joints', though in reality, unguligrades just have a proportional difference (their 'ankle' is a toe-joint, their 'backwards knee' is the ankle, raised but normal , their actual knee-equivalent is tucked up almost into the torso with the hip up well above that - in the rear it's almost 'shoulder' height, though the "front hip" 'shoulders' are only half way up their trunk, typically, with far shorter humeruses/humerii than femurs
They look backwards if you don't notice the humerus/femur element of their limb and extrapolate downward without realising the offset of joints you think they are from actuality.
There's also "overextension" of the knee (or other hinged joints), which can be a result of deformity or just well-trained contortionism (probably easier if you're already prone to lax ligaments), and enables (or maybe forces) humans to 'bird walk' (bird legs are similarly arranged to ungulates in their digitigrade manner, most easily noticed in long-legged birds like waders or specialist runners).
I think there have been case of true reverse-knees in a particularly severe form of birth defect, but often that comes with other issues (e.g. paths of veins and arteries through the legs).
In mythical creatures, though probably inspired by ungulates (the god Pan) or overextensive/contortionist abilities (Japanese mythologies seen a lot in anime and film) I suppose it would 'just' be "legs backwards on the bodyplan" (but feet, if plantigrade, still pointing forward). It's not like they need any explanation why they're so different from the base quadraped/biped theme only rarely seen changed (e.g. some arboreal swingers having more shoulder-like wrist joints, for better twistability of their gripping hands).
In imagining how to fit armour to 'reverse jointed' limbs, you probably have to consider whether there'd be a poleyn ('knee cap' element of full armour, a couter being the elbow equivalent) on the front-but-inside of the reverse knee, or would that interfere with movement? It's a tricky point to protect, the inside of a joint (knee, elbow, armpit, groin, neck) without creating loss of flexibility in the process, like some shin/foot overlapping armours that all but imobilise the natural flexibility of the ankle.
Then you have to draw it. As said, with a thought of portability. Perhaps easier with vectorised/skinned 3d art (already able to deal with arbitrary rotation my linking the limb mesh backwards?) but not so easier in this case. No easy one-solution-fits-all. At best, double up with separate overlay rasters to cater for the unarmoured (or otherwise 'not clothed') limbs they overlay/replace with armoured(/clothed) versions. Lot of work, just for an edge case. Might not be top priority for a while, certainly.
some various typos corrected