Santa costume legs was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The santa costume legs are part of the santa suit received from the 2008 Christmas Event. This item is unobtainable for players who did not complete this event. Players who have lost this item and want it back can go to Diango in Draynor Village, who will replace any lost event items for free.
During the 15 Days of Christmas in 2013, this item, along with other Christmas-themed equipment, were given the Hybrid style of combat, and stats that scaled with the player's levels. This was so players could use these items in combat to assist in hunting for festive crackers during the event.
When a player wears the opposite gender's costume legs, they will immediately change to match the player's gender.