Another member of the group is described as shorter, of stocky build, with black hair and wearing a red and white T-shirt.
The attack happened at the junction of Albion St and Boar Lane at around 3am on Sunday.
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 inquiries 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 Crimestoppers anonymously on 0800 555111.