Sick Sick? Or normal age-related issues?

No, I don't think it's your over-active imagination. The slight limp might indicate something like arthritis, and the pain could keep him from moving enough to get to the food pile often enough to maintain his weight. The vitamin C could help with his overall appetite, but I'd want to take him in to a good exotic vet for a checkup and for x-rays.

Don't bother with a small animal (dog and cat) vet -- most of them can't adequately treat guinea pigs even though they say they can. You don't say where you are in Tennessee, but UT Knoxville has a good exotic clinic, and I'm sure there are others in Nashville and Memphis.

@ bpatters: Thank you for your quick reply. I've made an appointment for tomorrow afternoon with my cavy-savvy vet who has met Tribble before. Funny thing is, I took Tribbie to the vet back in late November last year for one of his bouts of "seeming off." The vet did an exam and found no signs of arthritis, though he did not feel it was necessary at the time to do x-rays. He said he didn't feel like there was any physical discomfort (during the visit, at least), but he gave me an appropriate dosage of Metacam to use if I ever felt like he needed it. Is 3 years old enough for an active boar to start developing arthritis?

During the evenings, I let the boys out into my pig-proofed bedroom to run around and they chase after me as I drop their greens on the floor. Except for the days he seems "off," he runs around, orbits my feet as I drop lettuce, stands up and begs for his food, and shows no signs of discomfort at all. Same way with the morning feedings. It's very perplexing.

Yes, it's possible for a three-year-old to have arthritis. They're usually older when they develop it, but can be younger. As long as they don't sedate him for the x-rays (they shouldn't need to if they've got newer x-ray equipment), there's no risk involved at all.

Three years old isn't that old--these days pigs are living upwards to 7 or 8 years.

Has his weight gone down? Do you weigh him weekly?

What brand of pellets do you feed? How does he do with carrots, or apple slices with the peel on? Does he have any difficulty with those?

@foggycreekcavy: Yes, I do weigh weekly, sometimes more often. He's lost about 2.5 -3 oz slowly over the last few months, not very dramatic but gradual. I feed 1/8 cup (2 Tbs.) Oxbow adult food to Tribble daily; his cagemate only gets 1 Tbs daily cause he's a little plump. I've watched him eat and he doesn't have any difficulty with harder foods, not even those rock-hard Oxbow Natural Science Vit. C hay tabs. Vet peeked at his back teeth during the visit yesterday and didn't see any signs of malocclusion.

Update on Tribble: Took him to the vet for a check-up yesterday afternoon. Vet did not seem too concerned about his "3% weight loss" since he was last weighed at the vet in late November. Apparently, he weighed in at 2# 10 oz that visit and I weighed him at 2# 8.8 oz this visit. This seems to me an unfortunate mis-representation though, since he has traditionally weighed close to 2# 12 oz most of his adult life! I still feel that he seems thinner (but not very underweight) to me than he should be. As for the occasional irregular stools, the vet asked me if I had done any diet changes. The only thing I could think of was that I received a shipment of hay from KMS on 12/22, which is different from our normal hay supplier, Sweet Meadow Farms. I've also been giving them small amounts of storebought APD hay during floortime in the evenings because they go nuts for it. I do weigh my pigs weekly, and looking back at the recorded weights, it looks like Trib's weight really started slipping right around 2 weeks or so after this shipment arrived. The hay itself seems perfectly wholesome, although it is REALLY dusty. I've literally been having to shake the hay out as I feed and have been clearing out handfuls -- actual handfuls! -- of dirt off the top of the bale. Now that I think about it, I also feel like I've been seeing fewer of the irregular poops and more normal ones in the evenings, after they've been out for floortime enjoying their APD. I also recall distinctly that they did not seem as excited about their new KMS when I first introduced it to them as I thought they'd be, which surprised me as I've heard great things about it in general.
Vet still didn't want to do x-rays yet as Tribbie was not painful at the visit and in any case, "x-rays aren't really going to show arthritis unless it's already very advanced." As a vet tech myself, I know that this is generally true, so I did not question passing on x-rays this visit. I agreed with the vet to do a week or two of Metacam to test for any possible arthritis pain. If he doesn't seem to be improving after that, he agreed to do x-rays and sedate for bloodwork. Currently, Trib seems to be feeling frisky and fine.
So starting tomorrow, the treatment I'm doing is follows: Vit. C supplement daily, Metacam daily, going back to SMF hay.
What do you think of my/my vet's treatment? Are there any possible side effects of Metacam I should be on the lookout for? Has anyone else ever heard of switching the hay causing gradual weight loss (and does the approximate time frame make sense for this possibility)??? Thanks so much!

We switch hay all the time here at the rescue and while some pigs may eat less of some kinds, they usually don't lose weight.

I was wondering if he is a satin as well as a teddy. Osteodystrophy would explain the limping and arthritic behavior. Some pigs who are not satins can have it.

I don't think he has any satin in him. The thing is, I'm not convinced that this is arthritis. I should clarify that although on his "off" nights I sometimes feel that there might be a limp, it's never been anything obvious. It's more that maybe once a week he doesn't feel like coming out to play in the evening, and getting to rest somewhere warm and comfortable seems to help him feel better. But the rest of the time he is a very active little pig. He's been on the metacam daily for about a week now and so far I haven't really seen any difference in his behavior. He still seems happy overall.

I'm starting to think maybe the weight loss I'm seeing is related to his decreased pellet intake. I'm not sure why, but he doesn't seem as enthusiastic about eating his pellets the last month or two whereas he used to gobble them up. I don't think it's a tooth problem, as the vet checked his teeth and he doesn't have trouble eating anything else. I can still get him to eat at least a tablespoon a day, sometimes 1 1/2 tablespoons, but it takes him a while to finally decide - reluctantly - to go and eat it. Is there anything I can do to get him back on his feed?