I have a single pet sheep called Colin. Very much a pet. He thinks he's a dog.
Colin was an orphaned lamb, who had fly strike when he was small, and didn't get big enough to send away to become chops with the other lambs on my uncle's farm. (As well as the treatment he had meant he was in meat withdrawal at the time) He followed my cousin around all the time, didn't herd with the other sheep and was a bit of a loner. When she was headed back to Uni, she wanted us to take him on.
Colin gets on well with my dogs, and plays with them. He's happy enough on his own. He got sick last year (sheep were created to escape or die, right?) and the vet came out and I asked if we should get him a couple of friends, but he said (apart from being ill at the time) he looked happy enough and was pretty spoilt!
Colin stands guard for my ducks and chickens. We have not lost any to foxes (or seen the fox within our boundaries) since he's been around, patrolling the area. (and it was a regular occurrence beforehand)He's just a mix breed sheep, and he's pretty big now, but I think it's all wool. I think at the moment he weighs about 40kilos as he had a limp the other day so I had to tip him up to check his feet, and he weighed so much more than he did last year after shearing.
My husband and I have learned how he communicates that he wants to play, or have a scratch, or some food. And he comes when you call him.
Colin was born in Feb 2011 and we took him on in September the same year, he was a stressed little sheep, with patchy wool. Now he's confident and playful and knows that when you say no, it means back off. (He went through a phase of headbutting my leg, and it hurts if he catches my knee!)
I wouldn't have another sheep though, because he has problems so often it's crazy. Just when you think you're on top of worming, fly treatments and everything else, he does something stupid like getting stuck in some brambles in the middle of the night.
And playing hide and seek with a white sheep in the snow is fun too.
But he makes so much poop, I wouldn't want more than 1!