Donkey Tail is a common name for Euphorbia myrsinites, which is a lovely little rock garden plant. It has evergreen foliage with a hint of blue, and grows bunches of yellow flowers in the spring.
If you let the seeds develop, you may have a problem. Donkey Tail plants seed profusely. About two weeks ago I weeded around 200 of them. It was so hot that I wiped the sweat from my face and got some of the sap on my face. Within minutes my face and lips were burning terribly. It took around 15 minutes under running cold water to wash it off, but I was lucky to not get it in my eyes as the sap can cause temporary blindness. After two days the burning feeling had cleared completely, but I strongly suggest using gloves whenever you handle this plant.