Wonky Donkey, as well as being the punchline to a much-loved joke, is a game featured on Ant and Dec's Saturday morning TV show on ITV, aptly named Saturday Morning TV, or SMTV for short. In general, the contestant is shown a creature with some sort of distinguishing feature, and has to describe what he/she sees in the form adjective noun, where adjective rhymes with noun.
For example, a monkey wearing shades and getting down to a disco track might be a Funky Monkey. And so on. You get the general idea.