Animal psychology is still too primitive for them to question existence, but among certain 'higher' species like apes and dolphins, we can already see a developing social structure as far as some 'leaders' are more or less authoritarian. There are also superstitions emerging amongst some troops which leads me to believe that with enough time, other social species will develop religion.
With that said, what is heaven? Is it just a utopian dream that we as humans created? If not, then why would only humans be allowed?