We know that under most circumstances we do everything we can to prolong life and certainly prevent death for people, but what about animals? Our goldfish was clearly on his way out of this world – bent over in half, not really swimming, but occasionally moving around, not quite dead – and we weren’t sure if we should put him out of his misery or let nature take its course….
Would the correct thing to do change depending on the sophistication of the animal? For instance, a dying cockroach vs a sick dog?
Please see the following link http://dinonline.org/2015/12/20/killing-sick-or-injured-animal/ where a similar question was discussed. Regarding the idea of prolonging life, we find this idea regarding people, essentially because the person can still fulfill mitzvos and earn a little more eternal life. This however would not apply to animals.