The Holy Mackerel is the motorcycle Reginald Fletcher used every time he did stunts such as jumping over a whale, his mother's tea society and the Queen Mum. Reginald attempted to jump McGregor's Gorge with the Holy Mackerel on several occasions, but was unable to on account of sudden rain caused by his new theme song.
Later, the motorcycle was retired and used a lamp in the Flynn-Fletcher House. Phineas and Ferb refurbished the motorcycle and upgraded it so that Reginald would be able to make one more attempt at jumping a recreation of McGregor's Gorge in their back yard. ("The Flying Fishmonger")