General: genital slit
On dolphins, orcas, whales and other marine mammals, this is a cavity where the penis remains when not being used, mostly to preserve hydrodynamism.
On anatomically correct scalies and avians, this should normally replaced with a cloaca, which encompasses the anus as well, but many artists prefer to draw both orifices separated. Use this tag when it is the case.
Should the seemingly most sensible location of the anus be blocked from view (usually simply caused by a frontal pose, but it can be an object) while also not being visible inside the slit, then you can still use this tag as the default option.
If there is, however, a clear view of this area and it is featureless, then we can assume the anus is inside the slit and should refer to it as a cloaca.