Canon patent application US20190094490 describes the optical formula for an 8-15mm fisheye lens. Given the back focus distance this is most likely a lens for the EOS M system.
From the patent literature:
In order for a zoom lens to have a wide angle of view and achieve high optical performance across the entire zoom range and the entire object distance range, it is important to appropriately set the elements constituting the zoom lens. For example, it is important to appropriately set the zoom type (the number of lens units and the refractive power of each lens unit), the lens configuration, and the lens unit selected for focusing, and so on.
If these configurations are not appropriate, achieving a wide angle of view will increase the size of the whole system and also increase the variations of various aberrations caused by zooming and focusing. It will therefore be difficult to achieve high optical performance across the entire zoom range and the entire object distance range.
For example, for a fisheye zoom lens having an imaging half angle of view of 85 degrees or more, a meniscus lens having an extremely strong negative refractive power needs to be disposed closest to the object side within the first lens unit in order to take light rays into the zoom lens from such a wide angle of view. Generally, lenses in a first lens unit not only have strong refractive powers but also have large effective diameters. Thus, when it comes to a fisheye zoom lens, its first lens unit is heavy and large as well. Then, if the entirety or part of the first lens unit (the closest lens unit to the object side) is used as a focus lens unit, quick focusing will be difficult.
Source: Canon News