I've posted about my ancient bunny (12!)before when he threatened to be ill a few months ago.
Last night his lungs filled with fluid in 45 minutes and his temperature dropped to 34. The vet said it was pneumonia and he wouldn't make it.
He had an antibiotic injection the day before and liquid anti's that day so he was already a bit snuffly.
He spent his last day yesterday having a really good day. He was in the kitchen overnight (it had turned too cold for him to be outside) and I'd put him into his run to sit in the warm sun. I brought him in at six and he ate plenty of food and scratched around.
DH went to bed at 9 and he was still happily woffling around but when I came in at 9.30 he had fluid coming out his nose.
I rushed him to the vets where he was given a strong painkiller and he gradually went to sleep in 10 minutes - in that 10 minutes he was snuffling and rubbing against my hand. I believe knowing I was there 
Then he gave him the lethal injection and my lovely boy was gone.
He had a wonderful life - he escaped earning him the Houdini bunny nickname, he chewed threw a harness and hopped into the middle of a country road where he went to sleep - man in car drove up to him and I ran out the house to pick him up and have the puzzled driver say "I knew that wasn't a wild rabbit, he tried to hop up to the wheels". He dug his way out a few times of his Eglu - and even two weeks ago he was digging a warren in his run.
He was a very lucky and very loved bunny.
He is missed today by DD (who has gone to school sobbing), DH, our Springer who licked him to say goodbye and our 4 cats who have all visited his cage to sniff him. He loved hopping up to the cats and dog to say hello.
And of course me - he has seen me through 2 marriages and 7 house moves.
RIP Hot-Cross Bun.