The Pigskin Club of Washington gave member Marlon Barry a chance to function as mayoral nominee last night by leading a tribute to Liz Young, U. of D.C.'s All-America sprinter . . . Tonight at the National Democratic Club (the old Rotunda restaurant) at 30 Ivy St. SE, down the Hill from the congressional office buildings, Muhammad Ali is a promised guest at a fund raiser for Rep. Ralph Metcalfe (D-Ill.), the 1936 Olympic gold-medal trackstar: 6-8 p.m.

How about old Claude Humphrey, ofttime All-Pro still a regular on the Atlanta defensive line, up and retiring from the Falcons because "the time has come," he can't get up for NFL Sundays anymore? . . . Shuttling back, though, yesterday was Dan Pastorini, the Oiler quarterback who stomped off the Houston practice field the day before yelling, "Trade me, trade me, trade me!" He still has a mad on for offensive coordinator Ken Shipp over who should call the plays, and who was responsible for calling the lemons Sunday against the Rams - but he doesn't want to hurt Coach Bum Phillips - "It wasn't fair to Bum," he said of his walkout.

A fillip for the 1978 Washington International Horse Show at Capital Centre Oct. 22-29 - announced by Britain's Prince Philip: the President's Cup, premier event of the annual equine extravaganza here, has been selected as first event in a new World Cup Series of horse show jumping. Points will be accumulated at six U.S. competitions, top riders at season's end to go against European and Latin American equestrians selected in similar series.