If someone has recently lost their pet and you want to console them, here is a simple template you can use to write a condolence letter for a pet loss.
My deepest condolences for your loss. I knew how playful and fun your pet was. I event witnessed all the times you spent with each other. Also, I knew he was such a big part in your life. In fact, I have never seen any pet so loyal to his master before.
I am just here in case you need anybody to talk to in this difficult time. In life, there are many happy and sad times and this is definitely one sad time that will have a big impact. I know it will be one of those events that will stay in our minds for a long time.
I am your friend and I won’t hesitate consoling you when you feel down.