I am very sorry you have to do that but you don't want them to suffer that is for sure. We ( well my mom) have lost 3 horses since November 2008. Two of them had cancer, one of which had a very fast, aggressive kind that the vet said in his 30yrs has never seen. And one tore her tendon in her rear leg which became infected and was so lame she couldnt move. And I have been there when they all went down and when they were burried. I warn you though if you have never experienced it...... its hard , very hard. Depends how you are with things like that but you might want to say your goodbyes and then leave it to the Vet. Again I am very sorry.