UK BAP lists the Nail fungus as probably the rarest fungus in Europe restricted to the New Forest and only a few other places in the south east of Europe. Listed as Endangered on the UK Red List and Indeterminate on the IUCN Red Data List.
The fungus grows on horse and pony dung particularly where they have fed on unimproved grassland or hay meadows. Once widespread in UK its rarity is due to changes in agricultural practice - both the loss of unimproved grassland and heathland; and the reduction of horse use in farming. For more information see:
This specimen was found on Longcross Plain Nr Fritham. However, I can't claim to have found it, as it was spotted by a friend whilst we were out walking.