M for Mystery?!

I was feeling a bit crabby this evening.  In funny coincidence, Vaughan brought in a new visitor from the doorstep.

Even with several hours of rain (and the nearby bore-drain), we weren't expecting to find this fellow so far from the seaside!

We're not sure what he/she is, aside from the obvious broad category.  There is a very distinct M on the back plate though - for ... mystery?!