I AM LIVID! he gave my dog away to his baby mother

Long story short my fiancé at the time bought my daughter a puppy for Christmas  2015 and we named him Cooper. Now we were in a long distance relationship and he brought the dog from Jersey. Shortly after getting the puppy we found out I was pregnant! Fast forward to December 2016 where our baby is 4 months and I'm having to juggle two kids and a hard headed untrained dog. I wanted to give Cooper away because it became too much so my fiancé said he knew someone that wanted Cooper. When we visited I brought cooper to jersey because he always traveled with me. Well my fiancé gave him away when I wasn't there and wouldn't tell me who he gave him to. I was so angry as I wasn't really going to give him away so my fiancé did it out of spite! So from January to now my 8 year old would write letters to Cooper and cry about missing him which my fiancé witnessed it  and he still claimed he didn't know where Cooper was at! 
So today we were on FaceTime and I randomly blurted out "Where is Cooper" and his 6 year old son said "He with Me" now my ex fiancé gave his son a look and told him to say he was kidding but the look was one of shock that he said he had Cooper! Now his mother and I don't get along and he knows I would have never approved him giving Cooper to her. I am fucking LIVID and I hate him!!! He really hurt us especially my 8 year old and he lied over and over. Why would a 6 year old just say that if it wasn't true??? Am I wrong? I want nothing to do with him now and we have a 8 month old together? He claims that I should have never said. I wanted to get rid of Cooper! Now he is ignoring my calls and told his son not to answer my calls either