Dear, [friends name]
My dog, [Dog's name] ran somewhere far away, it was stressful because we didn't get [him/her] a collar yet, so someone could have found [Dog's name or him/her] and not been able to return them. My family and I couldn't find my pet for 3 days, i was really sad because I didn't know if [he/she] was coming back. Then my mom came to tell me something. "[your name], we have found [dog's name]! It turns out you never brought him home from your friends house!" She said. I laughed because I remembered I brought my dog to my friend's sleepover and forgot to take him back! I don't know how I could have forgotten but i was happy i had my dog back!
Love,
[your name]
------------------------------------------------------------------------------------------------------
Hope I helped! Let me know if there was a problem! Don't forget to add him/her your pets name your name and your friends name on your letter!