"I knew that."
*to self* "I did not know that."
Marion was voiced by Dan Tetsell, and was performed by Warrick Brownlow-Pike.
The first Marion puppet looked like a stripey mouse so the creators remade him, his body was supposedly based on James Corden topless and his voice came from when one of the creators did an Omid Djalili impression.
In the unaired pilot episode, 'We Are Mongrels', he appears differently than in the series, and also had a different voice actor.