The dogs featured in the pictures at the end of the movie (during the credits) were winners of a photo/essay contest hosted by www.dogster.com. Members had to submit a photo of their dog along with an essay explaining why their dog is a hero.
There were a total of 4 dogs to portray Rexxx, both to accommodate their energy levels and because of their specialty "moves."
As of 2022 this is the first and the only live action film from 20th Century Fox where the opening credits and title only appeared at the end. All the previous movies and more movies after that would continue to have the opening credits and title in the beginning.