Mabel is or was a female non-anthropomorphic whale, who may, at one point, have been a ghost.
A relatively small whale with a friendly disposition, Mabel was trained by pirate Ferdinand Blackduck in the 18th century to respond to whistled directions, and made into the powering-source of his ship, the Lucky Rocket; fastened under the boat, she could propel it even with no winds, giving Blackduck a clear advantage over any other ship of the era.
After Captain Blackduck became a ghost in his quest to find a descendant who'd carry on his plundering legacy, Mabel also seems to have become a spectre, as she was still by Blackduck's side in the year 2000, looking not a day older; though you certainly couldn't tell by sight, as she still interacted with the physical world with no trouble, suggesting she may have still been flesh-and-blood but been turned immortal (possibly through Blackduck's arcane powers).
It is probable that even after Ferdinand retired a short while later, Mabel stayed behind, now as the pet of his heir Michael, though it is also possible that she moved on like her master and Michael found and trained a whale of his own.
Behind the scenes
This whale was only ever seen in the 2000 story The Last of the Blackducks.