They are more like normal types, but just dark.
I don't know, it would honestly be a waste a space, in my opinion. I'd probably prefer Zorua/Zoroark to be honest. At least it would give Ash a gimmicky Pokemon.
Or how about Houndour/Houndoom, I'd much prefer them over Poochyena, sure maybe Poochyena is cute and stuff, but I don't think that alone is going to work with Ash.
It's bad enough that we already have a waste of space in Oshawott.