There was a scene in the Ren and Stimpy Show where this yak has a
problen with Ren, so he starts punching him up. I think the yak at one
point says, "And here?s one for your mother", or something like that.

Which episode was this scene from?

You're misremembering: the Yak actually beats up a sandwich, not Ren.

The episode is from Season 2 of the show, and the episode is entitled
"Mad Dog Höek/Haunted House." The second half is the one that
interests you. It's about
a hapless ghost that unsuccessfully tries to scare Ren and Stimpy. In one part,
he gets made into a sandwich. The Yak is, for some reason, present to
"tenderize" the sandwich. While "tenderizing", he shouts the "here's
one for the old man" type lines you're asking about.

Episode summary: http://en.wikipedia.org/wiki/Mad_Dog_H%C3%B6ek/Haunted_House