What is this growth on a Rosa rugosa stem?
This looks like a gall. At first I thought that it was the common rose bedeguar gall, also known as Robin's pincushion gall which is caused by Diplolepsis rosae, a small wasp. However, there is a related wasp, the spiny rose stem gall wasp Diplolepsis spinosa, which attacks Rosa rugosa so I imagine this is what you have here.

Incidentally, you are probably wondering what bedeguar might mean. According to the OED it derives from a French word bédégar which in turn comes from the Persian bād-āwar ('wind-brought’).