它 can refer to any kind of animal or inanimate object so it could be 它不是我的狗。
Yes, I don't know why they say it's wrong..
Because the English word is "this" not "it"
I thought that too but it shows as incorrect.
What is the measure word for a dog?
More common is 只 (Traditional 隻)，and depending on the area, 头、条 are also common (Traditional 頭、條).
"zhe jiu bushi wo de" can also have the same meaning, although maybe it can be read as "This dog is not mine"
Just a friendly reminder that the pinyin for dog is gou not jiu.
这个狗不是我的 is accepted (notice the 个)