Does it matter or not if you kill a mating Bull Netch (or the other, which ever is bigger)?

doesn't matter, kill all of them if you like.

Well, I wasn't sure. I got a box on the bottom right saying 'You've killed a Mating Bull Netch'. Thought it might have been one of those incidents where it could alter something..? Ah well.

i believe there are a couple netches like that in the game. did it pop up on the bottom of the screen journal updated? cause there is 1 netch i know of that you have to kill by a horipilation fork in order to get something acomplished. along with a white guar you need to do something with(wont spoil that one) but yes there are beasts throughout the game you are either not supposed to kill or you are supposed to help out

There are two mating netches in or around Gnar Mok(?) that are part of a local/imperial army quest. If you're getting into the imperial ranks eventually the head imp over at the fort outside of Balmora sends you after them.