Monday was a big day at the races for Lord Killanin, president of the International Olympic Committee, with his son the jockey winning the Irish Grand National and Lady Killanin presenting the trophy. Yesterday, the IOC guardian of amateurism was cheering his professional athlete son to another victory at the same track, Fairyhouse, when . . .

Heart attack.

Lord Killanin was rushed to St. Vincent's Hospital, Dublin, and taken to intensive acre. At last report, Killanin, 62, was "resting comfortably." His secretary said his doctor had been with the jovial successor to the late, strait-laced Avery Brundage at the track and believed there was little danger he would not recover.

Killanin, a onetime journalit named Michael Morris, saw jockey Michael (Mouse) Morris win the Grand National aboard Billycan, an 8-to-1 shot; he was stricken as young Morris scored on Bunker, 2-1 favorite, in the Power Gold Cup.

The heart seizure was described as "mild." Is any coronary mild? . . .

The Detroit Tigers played Willie Horton in left field opening day last week - his first appearence at the defensive position since July 7, 1974. Last hurrah? You bet. Yesterday, while rookie Steve Kemp was laying a three-run homer on Toronto by way of emphasizing his claim on left field while Rusty Staub handles Horton's longtime designated-hitter job, the Tigers were dealing Willie away.

The Texas Rangers traded righty relief pitcher Steve Foucault, 27, with career 25 W, 25 L including 8-8 last year, for Horton, 33 , a .276 lifetime batter with 262 home runs, 886 RBI, all with Detroit - his hometown, where he grew up - and a history of injuries, leaving little mobility in his knees. Once, after a season under Billy Martin, Horton said he had played his last in Tigerville; going on five years later, it has come to pass - but only a day after Horton lamented that bench duty is "like slow death for me. If you can't use a person, you ought to let him fo someplace else where he can play." If Texas lets him play, it is probably the forerunner of another Brad Corbett trade for lefthanded pitching; or will the Rangers now use Horton as bait?

NBA first draft choice, continued: The Indiana Pacers will make a substantial offer, possibly including leading scorer Billy Knight, to whichever team picks Indians U.'s 6-foot-11 All-America, Kent Benson, Pacer coach-GM Slick Leonard ventured, "I really feel if Kansas City wins the flip (Friday for No. 1 selection in the June 10 draft), there is practically no chance of getting Benson. Possibly if Milwaukee wins, there is a chance. Either way, we'll contact 'em and make a proposal . . . We'll do everything within reason to get Benson.I talked to him on the plane last week, and Kent wants badly to come later" . . .

If he can't get the Hoosier star, Leonard says, he would try for Darryl Dawkins from the 76ers or Marvin Webster from the Nuggets. And, if nothing materializes, hope Len Elmore can come back healthy . . .

Elmore's old Maryland buddy, Mo Howard, had a little something to do with settling the Cleveland Cavaliers into sixth place in the NBA East, setting them up as Washington's first playoff hurdle. In a late-season game at Richfield Coliseum, the fans shouted at New Orleans coach Elgin Baylor, "Put Mo in! Put Mo in!" Baylor put Howard in for the fourth quarter and he popped eight points to help Jazz partner Pete Maravich beat the Cavs, 92-87. The former Terp guard, drafted No. 2 by the Cavs in 1976 but pinned to the early season bench, then released to umemployment until the Jazz picked him up two months later, said, "I have no bitter feelings. I was sorry to leave Cleveland because the fans were great to me. But I am happy that I made a contribution to beating them" . . .

The Jazz are out, but Maravich makes the playoffs - CBS-TV has added him to the commentary crew . . .