Plants don't come much rarer than this one, or much more underwhelming either. This is Lady Clermont's Spleenwort Asplenium x clermontiae. This is the only one in the UK and is visited by keen botanists from across the country. While it is only half a mile along the road from our house, this is the first time I've seen it, despite looking. It has taken a local expert to show me its exact location, on a wall along the coast road. To its right is the much commoner Maidenhair Spleenwort, one of its genetic parents. The other one is the Wall Rue. Apparently these natural hybrids are so rare as the parents are a different genus, or something? [Apparently not - see comment]
Unlike hybrid birds, I think plant people can tick this one without guilt, so its now on my list.