The killer fish - thought to be piranhas - were left behind on a seat and retrieved by a driver, who failed to identify them and took them home.
But his good turn ended in horror when he tipped them into his goldfish bowl and the stowaways promptly ate his own pets.
The UK’s largest coach operator revealed the weirdest items left on board over the past 12 months in a drive to remind summer travellers to remember their bags.
Another customer left a case of saucy ‘adult’ items - including whips and handcuffs.
The past year has seen all manner of items mislaid on-board vehicles including:
An urn left in the hold - thankfully quickly reunited with its owner; a wedding dress and three bridesmaid’s dresses left by a forgetful bride the day before the wedding; black and white photographs from the 1940s dropped by an elderly war veteran and later reunited with the family and a pet cat left at Cardiff Coach Station - later adopted by one of the station managers.
A giant game of snakes and ladders, a toilet seat and a hat stand are also among the forgotten items which remained unclaimed.
National Express staff have become accustomed to seeing everything under the sun left on board vehicles, with previous items including a live crab, a false leg and a bag of Borat-style ‘mankinis’, which were lost en route to Glasgow.
The coach company also sees thousands of more mundane items handed in to lost property, with the most commonly lost items including glasses, walking sticks and mobile phones.
Lost property manager Claire Horvath said: “From the mundane to the bizarre, we really do see it all.
“We’ve enough phones in lost property to open our own phone shop but every now and then we get something that really leaves you scratching your head - you just don’t expect your average coach passenger to be carrying whips and handcuffs.
“It’s incredibly satisfying reuniting people with lost items but we would much rather people didn’t have the stress of losing them in the first place and want to remind holidaymakers to make sure they have all their bags when travelling this summer.
“This year it was particularly rewarding to get the urn quickly returned to its owner, who was incredibly relieved, but we do feel it may have been serendipity the ‘mankinis’ did not make it to Glasgow”.