The great flightless bird Gastornis, which is presented as a fierce top-predator in the show's first episode, has in recent years turned out to have been omnivorous, eating mostly plants. Hence, its portrayal as a fearsome carnivore is unsubstantiated.
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 Walking with Dinosaurs (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.
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.
Some of the animals were redesigned for the companion book and in promotional images. In particular, the Dinofelis cat looks almost entirely different, and the cave lion was also given a short, stumpy tail instead of the long tail it has in the series.
The animators used guide-hairs to create the fur and feathers of the show's creatures. These were single strands of hair whose animation the computer software copied onto the other hair strands around it. This process made animation much easier, as it didn't require all of the animals' hair to be animated separately, strand-by-strand.
Almost twice as many animators worked on the show as on its predecessor Walking with Dinosaurs (1999), due to the difficulty of animating mammals and other animals which would be more familiar to viewers than dinosaurs.
In 2001, the series was given a special broadcast on Active Digital's digital cable service, in which viewers could access a large library of additional info, such as fact files about the ancient animals, info about their habitats, as well as behind-the-scenes clips.
The series proposes that the great mammalian carnivore Andrewsarchus was a majestic-looking, wolf-like hoofed animal belonging to the Mesonychids, a mammal group related to whales. This idea was widely accepted back then, but nowadays Mesonychids aren't considered close relatives of whales, and Andrewsarchus itself is thought to have been more closely linked to the ugly, pig-like Entelodonts (which were distantly related to hippos and whales).