Police investigating an attempted robbery at a Melbourne railway station are appealing for help identifying a so-called “brontosaurus bandit”.

A man wearing a green dinosaur outfit is among a group of youths believed involved in the October 16 incident in which a 21-year-old man was threatened with a glass bottle.

Police say the Glen Waverley man arrived at Syndal railway station about 3pm and was walking through a pedestrian tunnel when a male in a group that had been travelling on the same train grabbed his backpack and pulled him to the ground.

When the man refused to hand over his wallet and phone he was threatened with a the bottle.

He managed to get away uninjured as the group began throwing other bottles at him.

The youth wearing the dinosaur costume and a female in the group returned to Syndal station and took a train to Glen Waverley.

Police say they want to speak with anyone who saw the incident, and have released CCTV images of the four youths including the one in the onesie.