Whenever one of our pets, rather cat or dog had to be put down, the remaining pet(s) always went through a period of grieving/mourning, it was sad to see this, as well as sad for us.
I've seen several nature shows that were about this topic, elephants especially, was one of them that amazed me, and I found very touching/sad.
I've also seen situations where a living animal will go over and nudge the one who had died, trying to bring it around. Yes, I truly believe animals have feelings, though they may not express them in the same manner humans, it appears to be obvious that there is some special connection they share amongst themselves.
I remember so clearly, the helpless look one of our dogs gave me as she glanced over to where the cat use to like to catch the sun's rays.
I personally find, losing a pet as saddening as losing a family member, after all they are family too.
Sorry to hear of your loss.

((((((((( fayerody and pets ))))))))))