An attemept at Terza Rima:

[Philatone]

hey billy

this feels as though it was fun to write, despite the difficulties meter may bring. some thoughts (haven't had a glance at the other comments just yet)

1st edit
My Best Friend:

A holstered tale between two nervous knees:...nice wordplay
O churlish cur, why bite the hand of care?...a bit formal for the story? personal preference; do like how you play on the old cliche
the lamb was not for eating if you please...."as" you please?

I call you friend and yet those fangs you bare.
We two, we have a shepherd's job to do,...would need another beat, but i feel like "We, two, have a..." has a stronger flow
you're not supposed to be a wolf, just glare...."just glare" feels tagged on to me

A year to wait til it becomes a ewe...a bit awkward. maybe "A year from now it would have been a ewe"
and then we'll strike, but silent with a blade,...if the suggestion above holds, could change this to "we would have struck"
or axe, and have more meat to place in stew.

But no you greedy bastard, had you weighed...liked this, takes everything back to reality in a practical, muted tone
the situation up you would have heard...the "up" struck me as a little odd
a whistle saying how you should have stayed. ...
say" wasn't my favorite choice for the "whistle"

Consider this a punishment deferred
and next time know that you shall be interred....first line I liked, the second line maybe a little rushed? I think something stronger may come to you

Original:

My Best Friend:

A holstered tale between two nervous knees:
O churlish cur why bite the hand of care?
the lamb was not for eating if you please.

I call you friend and yet those fangs you bare.
You and me, we have a shepherd's job to do,
you're not supposed to be a wolf, just glare.

A year or so when it becomes a ewe
and then we'll strike, but silent with a blade,
or axe, and have more meat to place in stew.

But no you greedy bastard, had you weighed
the situation up you would have heard
a whistle saying how you should have laid.

great tone and idea; i enjoyed reading this a lot. hope some of the suggestions can help