Dale Nixon is an alias that was first use by Greg Ginn when he recorded the Bass on the album My War by the band Black Flag.
Then it has been used by:
Brian Baker, when recording Four on the Floor with Dag Nasty, because he was under contract to another record label at the time.
Dave Grohl, when recording King Buzzo with The Melvins.
Kurt Ballou, when playing Toy Drums in the album Board Up the House for Genghis Tron.
Ryan Adams as the bass player in his own project Orion.
As the wikipedia article says, it is often use
usually to appear on albums without being in breach of record label contracts that stipulate label exclusivity.