Yaaaas! I have vague memories of this! Why does Cujo have a Werewolf form? Is he the only one that does?
When they were younger, there was a super soldier program spearheaded by the royal scientist at the time (here it was not Gaster, but the River Person, so he didn't have a name other than Royal Scientist). Gaster thought if he gave him Sans (Vanta), who was overwhelmingly fragile, it would give him a better chance at life. Basically he had the right intentions, but he pretty much sold his kid for science.
Papyrus (Cujo), who had basically been raising Vanta since he was a babybones, did not accept this. He ran away from home to track his brother down, and basically offered himself in exchange (because the contract said "Son of WD Gaster" but didn't specify which one, and Cujo argued that Vanta had too low of an HP to be useful for test runs bc he wouldn't survive a needle prick).
The experiments were a success! Except for the part where nobody could control the final product, making Cujo useless to the guard and therefore the program was scrapped due to too many failures. They tried to kill him, and he might have let them to end the pain, but Cujo caught sight of Vanta in a cage across the room and remembered what he was here for. He used the new power he had gained to kill the royal scientist and some of his assistants, take his brother, and flee back home.
He gained notoriety instantly. He had a hard time controlling the form, and much of his lackadaisical attitude nowadays is learned behavior--if he loses his temper. The monster may come out.