It appears that the function or meaning of these is not well-understood. Of course, both might have changed with time and place.
Nonetheless, it is a nice reminder that Medieval Europe (and Britain) was a much stranger and more complicated place than the what is usually imagined in popular entertainment.
There are a couple of other common church carvings: the green man, and the imp (or devil ) - the meaning of the former is obscured in the mists of time, some claim it to be a pre-Christian fertility symbol but it doesn't appear to be pre-Christian.
It's certainly true that masons had their fun: though, unlike some 'green men' the images of Sheela-na-gigs seem quite rough and amateurish suggesting to me they were not necessarily authorised adornments.
In addition to the normal grotesques and gargoyles. Any other common forms?
Nonetheless, it is a nice reminder that Medieval Europe (and Britain) was a much stranger and more complicated place than the what is usually imagined in popular entertainment.