Shrek never gives Eddie Murphy's Donkey an origin story, but one convincing fan theory might just reveal the fast-talking donkey actually came from Pinocchio. The first movie was adapted from the children's book Shrek! by William Steig. The movie version greatly expanded on the source material and told the story of the title Orge as he reluctantly embarks on a quest to rescue a princess. Shrek is accompanied on his journey by Donkey, who acts as comic relief and also imparts some wisdom when Shrek needs to hear it.
The combination of Shrek's witty script and great cast made it a huge success upon release, and the series would spawn numerous sequels, short films, video games and other spinoffs like Puss In Boots. Shrek 5 is said to be in development and while the involvement of the original cast hasn't been confirmed, they are tentatively expected to return.
Donkey is a role that was originally earmarked for comedy legend Steve Martin (The Pink Panther) but Eddie Murphy was later cast. The Donkey found in the Shrek! book was a relatively minor character, but his role was increased for the movie. While the origins of other fairytale characters seen in Shrek like the Gingerbread Man or the three blind mice are well-known, Donkey himself doesn't appear to come from any specific fairytale. That said, a fan theory first put forth by Reddit user YeremyV’s suggests the character actually came from Pinocchio.
Fans of the animated 1940 version of Pinocchio may remember the sequence where the title character heads to Pleasure Island, where wayward boys can drink, smoke and do as they wish without adult supervision. It's eventually revealed the boys are being turned into donkeys and sold as slave labor. Pinocchio manages to escape this fate, though his friend Lampwick is transformed.
Even now, the Pleasure Island reveal is a deeply creepy scene, and it also reveals some of the transformed boys can still talk. That's where YeremyV's theory comes in, which states Shrek's Donkey is one of these boys grown up. In his introduction scene, Donkey's owner is an old lady who tries to sell him, though nobody believes a donkey can talk. Pinocchio seems to be the only major fairytale featuring talking donkeys, and in an interesting coincidence, Pinocchio is also seen in the same line-up.
Shrek The Third seems to offer more clues, with Donkey alluding to being bullied and suffering wedgies when he was young - which only really makes sense if he was human at some point. When he and Puss later exchange bodies, Puss also has a hard time controlling his hee-hawing and talking normally, which Donkey suggests he'll learn to control after practice, which alludes to him having to do the same.
While a lot of fan theories tend to fall apart upon closer examination, Donkey's origin coming from Pinocchio tends to make sense. One flaw in this theory is that the filmmakers behind Shrek once revealed Donkey was originally intended to be a regular donkey who was cursed, but since the movie itself never reveals this, it isn't canon. Maybe Shrek 5 or a future movie will clear this up, but this theory makes too much sense to simply dismiss.