We are assuming this is the larval form of something, but having difficulty finding out what. It was found on Willow leaves and is approx 1cm when fully stretched out. It also seems to flick the 'tails' at the back up when turning or disturbed.
Strangely enough the last reply I made to the forum was about one of these caterpillars on a late instar. See 'Large Green Caterpillar identification please'. This is an early instar of the Puss Moth Cerura vinula on it's favorite foodplant. It's the only two tailed caterpillar I know.