I did a checkup on my guinea pigs...
I cut the fur off their butts, because they have thick fur which gets covered in urine. On Scruff, it blocks up his scent gland too. So, I cut it, because I don't want them getting fly strike. :/
I checked Cupcake over; she was pretty good.

She's a little overweight, and had quite a urine soak butt, but she was pretty healthy.
Then, I checked Scruff; he's not very good...

He has swollen feet, misty eyes, a blocked scent gland, he's balding, and he's really underweight - I can feel all of his bones.
I don't know whether or not to take him to the vets. They'd probably just put him down - he's about six years old, so he's not young anymore - but I don't know if he's in pain.