Only at 15 mm
The 8-15mm is a fisheye lens at all focal lengths. At 14-15mm on FF, it's a 'full frame fisheye' meaning the FoV fills the frame (you're getting an AoV of ~175°). At 8mm on FF, it's a 'circular fisheye' meaning a circular 180° AoV inset in the rectangle (black borders).
If you want rectilinear from the 8-15mm fisheye, you must de-fish in post (at the cost of corner sharpness).
The widest rectilinear Canon lens is the 14mm prime, Sigma has a rectilinear zoom that goes to 12mm.