The forest is home to various creatures. Of course there is the regular non-magical wildlife, but there are also Centaurs, Unicorns, Acromantula, Thestrals, Bowtruckles and likely many more creatures. It is also rumoured to have Werewolves, if Draco Malfoy is to be believed.
The Forest in Harry Potter and the Philosopher's Stone
In the first book, Harry, Hermione, Neville and Draco had to serve detention in the Forbidden Forest. They were tasked with aiding Hagrid in the search for a wounded Unicorn. Harry and Draco ended up finding the creature, but were also confronted with a mysterious hooded figure. The centaur Firenze saves them from it and explains that the figure is actually Voldemort, who is trying to gain eternal life.
From the Story
"The Forbidden Forest is on the edge of the Hogwarts hrounds, near Hagrid's hut. The forest, full of magical, mysterious and dangerous creatures, is out of bounds to any student unaccompanied by Hogwarts staff.
"Harry is nearly attacked in the Forbidden Forest, when he witnesses a hooded figure drinking blood from a dead unicorn. The centaur Firenze saves him."