Carrie, the chimp who plays Voodoo, is clearly a full grown adult chimpanzee. This is one of, if not the only time, where an adult chimp is used in a film due to their strength and aggression.
The script originally called for Link to be a chimpanzee, but when Locke got the role, Link was changed to an orangutan - although trainer Ray Berwick dyed his fur black and gave him prosthetic ears, to make him look more like a chimp.
Locke, the orangutan who plays Link, and Jed, the chimp who plays Imp, go in wheel chairs before they start filming to save their energy.