The great flightless bird Gastornis, which is presented as a fierce top-predator in the show's first episode, was actually omnivorous, eating mostly plants and nuts. Its portrayal as a fearsome carnivore is unsubstantiated. The filmmakers were aware of the debate about the bird's diet, but chose to go with the predatory hypothesis, reasoning that its beak was far too robust and strong for just eating nuts. However, Gastornis' beak was not hooked like the beaks of most predatory birds and it also lacked sharp claws. Chemical analysis of its fossil bones post-2010 also found no trace of meat in its diet.
According to the bonus features on the Walking with Monsters (2005) DVD, Walking with Beasts was the series the producers originally planned to create. They went with Nel mondo dei dinosauri (1999) first, though, as they thought it would be a bigger success, since dinosaurs are more well known to the public than prehistoric mammals. Dinosaurs proved to be a big hit indeed, so eventually the production for Beasts got green-lighted as well.
It is not unusual for the narrator to refer to the different kinds of animals not by their proper genus or species names, but rather with broader terms (like the name of the group they belonged to), making it hard to identify the animals more precisely. For example, the giant rhinoceros Paraceratherium is simply called an "indricothere", and there is also the bizarre plant-eater Embolotherium, which is gets to be named "brontothere". This is true for all the "Walking with..." series, but the naming method began to get prominent in this show. However it also produced several dubbing mistakes for other countries; some dubs did not translate these animal names at all, others added one or two extra letters to the end of the names, while some translations tried to use more exact names for some of the animals, mostly ending up with incorrect or out-of-use terms - this can be forgivable, since in some cases, like with the aforementioned Paraceratherium, there are a handful of different scientific names that can be used.
The roar of the entelodont took ten times more to create than usual, being a mix of, among other noises, donkey, snake and elephant sounds.
During the filming of the scene in which the Ambulocetus lunges out of the water at the Propalaeotherium, the jaw of the Ambulocetus prop smashed against a rock and broke. In the finished program, the scene was done with a CGI Ambulocetus instead.