I've noticed something else about the Giant Sheep. It's described as "larger than a horse" but only weighs 700-900 pounds, which is light for a horse. An average riding horse is about 1000-1200 pounds and a heavy horse weighs 1500-2000 pounds or even more, with the world record estimated to be 3,360 pounds.
Scaling up some regular sheep breeds to be 6 feet at the shoulder (the height that AD&D gives Giant Rams) I came up with figures ranging from ~1400-2000 pounds.
Dire Animals are often portrayed as burlier than their normal counterparts, so I'll say an "average" Dire Sheep is 2,000 to 3,000 pounds.
EDIT: Hopefully I'll whip up a Dire Sheep later today but I have stuff to do over the next couple of hours.
I've whipped up a Dire Sheep and amended the Weresheep.
I added an additional tweak to the by increasing its speed to 40 ft., since the original Giant Ram had move 15.
Oh, I thought this factsheet on Cotswold sheep was a useful basis for the size. I pretty much just doubled the dimensions of this large breed of sheep and added a bit for luck and direness.
I guess we should consider amending either the weight or the description of the Giant Sheep.
Either keep "larger than a horse" in the description by making the "A giant sheep is about 8 feet long and weighs 1800 to 2400 pounds" or keep the existing weight and make the description "This massive sheep is as big as a horse."
Either works for me, but I lean towards keeping the "larger than a horse" version.