The Moxx of Balhoon was a guest onboard Platform One.
The Moxx had a brother, the Elth of Balhoon. (COMIC: Reunion of Fear)
By the time he encountered the Tenth Doctor, the Moxx was a noted solicitor. (COMIC: Who Ate All The Biscuits?)
In the year 5 Billion, the Moxx of Balhoon was invited to watch the destruction of Earth, representing the solicitors Jolco and Jolco. He gave the Ninth Doctor and Rose Tyler the present of bodily saliva, and spat at Rose. He was brutally killed in the heat of the sun's expansion when Cassandra lowered the sun filters. (TV: The End of the World)
References
When the Tenth Doctor took the identity of John Smith, he drew a picture of the Moxx of Balhoon in A Journal of Impossible Things. (TV: Human Nature)
Behind the scenes
According to the non-narrative source, The Ultimate Monster Guide, which this wiki does not consider a valid source, his legs were crippled by a disease, so he travelled on a fast-moving anti-gravity chair. The chair also replaced his body fluids every twenty minutes so he didn't sweat glaxic acid. His favourite song was "Yap Cap Forward Bigga Toom Toom Toom".
According to Doctor Who: The Encyclopedia, he was portrayed by Jimmy Vee but voiced by Silas Carson.