Which, the Internet helpfully tells me, is the caterpillar of Orgyia antiqua, the rusty tussock moth, or, in German, Schlehenbürstenspinner (it seems to go for unwieldy names in both languages). I've never seen one of these caterpillars before, but they seem not to be so rare.
When searching for what this might online, I found a nice site with quite a lot of caterpillar pictures to help identify their species: Schmetterlinge Westerwald. There's 152 different caterpillars there, and the photos alone are worth a look to wonder about the variety of shapes and colours that these tiny critters have!
The one in the picture, by the way, got set outside onto our own morello cherry tree - where it will hopefully be okay...