Well, they do look a bit like the exposed tails of pangolins which have snuggled themselves behind the rock. They also look like some particularly large-scaled reptile sunning itself on a rock: the Australian pine cone skink Trachydosaurus rugosus comes to mind.
What are they? They are Euphorbia myrsinites, and in this case they are particularly well placed. Rocks set off this plant very well, although it does not require a rocky setting to grow well. This is a cold-hardy and easily grown perennial here in the greater Washington, D.C. area, although it does not seem to be long-lived. If the prospect of moving such a large boulder into the garden is daunting, try planting the plant on the flat with a mulch of crushed blue stone.
Wayne and I were visiting his mom in Bridgewater, Virginia earlier this week and these plants were seen on the grounds of the Bridgewater Home. Inside the Home we saw poinsettias everywhere; I wonder how many people realize that these "pangolin tails" are a close relative.