My friend has an old dog who has Cushing's disease (not mentioned, I don't think it's as common in dogs and cats as it is in herbivores?), basically no cartilage in his hocks, can't see properly, can't hear properly.

He's a happy old guy who demands petting from everyone (and I swear exaggerates his joint issues to get attention) so he's not yet a candidate for euthanasia, but I do kind of expect him to drop dead any time. Poor old Snickers.