Sunderland Magistrates' Court heard how Jackson Adams, 21, ran across the Stadium of Light pitch wearing the mask and put two fingers up at the away fans.
Mr Adams is alleged to have been poking fun at an incident from April 2013 when Newcastle fan Barry Rogerson was arrested and later jailed for a year for punching police horse Bud following his side's derby defeat to Sunderland.
According to the Sunderland Echo, Adams said: "I'm absolutely gutted about it. It was just excitement. I haven't been locked up in years. I don't want a banning order. I guarantee it won't happen again. I was locked up for six hours."
The pitch invasion occurred during the Black Cats' 1-0 win over Newcastle last season.