Then I heard every creature in heaven and on earth and under the earth and on the sea, and all that is in them, saying: “To him who sits on the throne and to the Lamb be praise and honor and glory and power, for ever and ever!” (Revelation 5:13)
It says in Revelation 5:13 that every animal praises God. This leads me to believe God created animals for the same purpose humans were created: to give glory to God (Colossians 1:16).
The instruction God gave to animals when they were first created was for them to be fruitful and multiply (Genesis 1:22, Genesis 8:17) in order to populate the earth, which was the same instruction God gave to humans (Genesis 1:28).
The only difference between humans and animals is God gave us the additional responsibility of being stewards of all the animals (Genesis 1:27-29) and of the earth (Genesis 2:15).