I have no pictures of my horses hindlegs but I have looked at them often and found that just like the forelegs are slightly valgus, so are the hind legs. In the case of hind legs one often speaks of cow-hocked. Here again, the "textbook" knowledge indicates that straight legs are ideal. However, I found this picture on the web:
This is an excerpt from a book:
It states: "Most breeders would add cow hocks to the list, but this
is incorrect. Horses are differentiated from other mammals precisely on
the basis of the fact that horse hock bones (and stifle joint structure)
force them to stand with their hocks pointing inward. In other words, a
horse that is not cow-hocked is not horselike.; in fact, a horse with
"straight" hocks as seen from the rear (figure 32b) is likely to move
with wobbly, pathology-generating hocks. There are degrees of cow hock:
if extreme enough to cause interference (figure 32c), the fault is a
serious one, but hocks that face in are not a fault per se."
I am reasonably convinced, my horses hind legs conformation is absolutely fine, normal and healthy. They look just like picture 32A above, maybe even a bit straighter than this example.