God, that sucks. What breeds and how old?
Mom's is 13, purebred lab, so it's not exactly unexpected. He apparently got hit by a wave of vertigo the other morning and my stepdad found him collapsed in the snow, suffering from hypothermia (they're in a part of Arizona that has things like snow and hypothermia). They've been trying to nurse him back to health but it's been more than a week and he still can't walk, so if things don't improve soon, well, that's no way for him to live.
For the first few years of his life, he lived with me and my grandparents in the house where I grew up.
Dad's dog is about half that age and so it's a much bigger shock. He started walking with a limp a couple of months back; Dad took him in and he was diagnosed with valley fever, so he's been on meds for that. But it didn't get better, it got worse, to the point that he stopped getting up to come to the door when I'd come by to visit. Over the last few days a lump started growing at the top of his leg. Dad took him in to the vet yesterday and they said it's a tumor and he's probably got a month to live. When I saw him yesterday, it was about the size of an orange. He can still walk but it's obviously terribly painful for him, and it takes a lot of coaxing even to get him to walk into the next room or out into the backyard.
My grandparents' dog died a few months back. So this could be all three within the span of a year. Two, at least, were old, but a giant tumor at the age of six is an especially hard thing to see.