The victim, a 26-year-old man, was attacked at the junction of Albion Street and Boar Lane in Leeds city centre by a man who was part of a group of about ten men - one dressed as a giant penis (similar to the costume pictured).
The victim received a shattered shin bone, dislocated and cracked knee and torn ligaments. He underwent an operation at the time and further surgery will be required in the future.
The attacker was described as white, about 6ft tall, with a shaved head. He was wearing a beige or white top. Another of the group involved was described as shorter and of stocky build with black hair. He was wearing red and white T-shirt.
The incident happened at about 3am on Sunday, April 2.
Detective Constable Gaynor Burt, of Leeds District CID, said: "The victim was very badly injured in this attack and will still require further surgery. While the nature of costume worn by one of the group might be amusing to some, we are hoping that mentioning it will jog someone's memory and assist in tracing those responsible for what was a serious assault that has left the victim with long-term consequences.
"We have been making extensive enquiries to identify those involved and would like to hear from anyone who witnessed the incident or who has any information that could assist in tracing the group involved, who were possibly a stag party or similar. The costume worn by one of the group is very distinctive and we think anyone who encountered them during the evening would recall it."
Anyone with any information that could assist the investigation is asked to contact DC 3146 Burt at Elland Road via 101 quoting crime number 13170151315 or call Crimestoppers anonymously on 0800 555 111.