A likely cause is a small piece of debris inside the camera, such as swarf: a thin curl of plastic, metal, or even paper left over from manufacturing. If that debris sat near the film gate or optical path, it could cast a soft dark shadow into one corner of multiple frames.

Because the shape appears in a consistent area of the image, it points to something physically inside the camera rather than a lab processing problem. It’s also less likely to be a lens element issue; damage to the lens itself would usually affect the image differently than a localized serpentine shadow.

Since a disposable camera is factory-sealed until processing, the debris was probably present from manufacture. In that case, the practical remedy would be to contact the seller or manufacturer about a refund or replacement.