Sebastian MuscleWhale is a toy created by Robbie in the 1999 Robbie the Reindeer film Hooves of Fire. He is a toy whale with three muscular arms (two where a whale's flippers should be and a third that pops out of his back).
After being employed at Santa's workshop, Robbie was in charge of painting dolls, but wasn't good at it and so was demoted to sweeping by the Head Elf. Later that day, the Head Elf comes to check on Robbie but finds him not sweeping, but sitting amoungst pieces of toys. Robbie reveals to him that he made new toys and shows him Sebastian and also OctoMonkey, Sebastian's enemy. However, the Head Elf isn't impressed and demotes Robbie to being a forklift.
On the day of The Reindeer Games, Robbie learns from the Head Elf that Old Jingle is trapped under his house and is forced to temporally leave The Steeplechase race. After arriving at the scene of the incident, Robbie tries to lift the house off Jingle, but it is too heavy and Robbie says that he needs a wedge. The Head Elf gives him Sebastian, but that is not enough and hands over OctoMonkey too, which helps Robbie pull Jingle from under the house.
A poster of Sebastian fighting OctoMonkey titled "Titan of Terror" is seen on one of the walls of Reindeer Lodge.