Screenplay ideas were being written dating all the way back to 1991.
This is the biggest animated film ever produced in North America. Over four hundred animators were put in a large building to work on the film.
In the part where the animals are in the garbage truck you can see a sign advertising Radio Disney and another advertising for Kodak. There is also an advertisement of a theater showing The Lion King (1994) on stage when they are being chased by dogs.
Kiefer Sutherland had to practice roaring like a lion. Instead of doing it in his house with his two daughters around, he practiced in his car driving on the freeway, completely unaware of a woman in the next lane watching him.
None of the humans' faces were seen throughout the movie.