Donald Michael "Donnie" Thornberry is a feral boy who (as revealed in the 2001 4-part TV movie, The Origin of Donnie) was raised by orangutans after his biological parents, a pair of naturalists, were killed by poachers. Before the Thornberrys adopted him, the special also reveals that Nigel and Marianne knew his parents Michael and Lisa. He commonly wears a pair of leopard-spotted shorts and he loves to eat all types of bugs. He babbles wildly and makes animal sounds (but nothing Eliza can understand), occasionally spouting blurbs of English or showing random signs of sophisticated education, such as table place setting or properly drinking tea. He might also know French, as during a lesson he stated that an apple was a 'pomme', which is French for apple. Donnie is four years old for most of the series but turns five in The Origin of Donnie. It was shown that while his parents were teaching orangutans sign language, he learned as well. Though only one sign was shown (family), it is unknown how much he actually knows.
Donnie Thornberry was discovered in Borneo by Nigel and Marianne as they did a show about orangutans. His parents were killed by poachers, so Donnie was raised by the gentle primates since he was a toddler. When the Thornberrys came along, his orangutan mother decided that they would be a good family for the boy. Donnie has a lot of energy and is hard to control. He is often put into the care of Debbie, who finds her little brother to be quite a handful. Donnie will eat practically anything, including bugs and plants, and enjoys speaking in his unique form of gibberish and doing "The Wedgie Dance".
Donnie is mostly fair skinned. He has dark brown hair. Donnie wears a pair of gold shorts with black leopard spots and no shirt or shoes. He has an abnormally large mouth with large sets of excess teeth, and his hair appears to be in the style of spaghetti strands.