1- All animals will be resurrected on the Day of Judgment.
2- Some animals go to heaven, like the camel of Prophet Saleh or animals used in Allah’s path (like horses for defending Islam or camels going to Hajj on them).
3- Allah grants a believer whatever he wishes in heaven. If a believer desires to be united with his pet in heaven, Allah will grant him that.
4- We don’t know. We just know God is merciful and he will do that which is most wise. One reason why all animals don’t go to the heaven that people go to is because heaven is like graduation. People worked hard and they earned it. Imagine going to your graduation and on stage they put animals with you (like a monkey wearing a cap). That’s insulting. Heaven is a place for humans, not all animals, even though some exceptions are granted to some animals.