When Marilyn Monroe began her career, she was warned about a scene-stealing co-star in a western she was about to do. "It's not him that worries me," she said. "It's those hammy horses."

Her remark tell us very little about the talents of that particularly rhinestone cowboy, but they accurately reflect another old show-biz maxim - as scene stealers, animals and kids leave the average actor chewing the scenery.As W.C. Fields said, anyone who hates dogs and children can't be all bad.

Perhaps that's why, in the past few years, most animal movies have been about less cuddly creatures, such as sharks, killer bees, giant apes.

However, horse movies are making a comeback. Although nowhere near the halcyon days of the late '30s when everyone from Ronald Reagan to the Marx Brothers was making the same "Let's-win-the-big-race-for-Gramps-and-save-the-farm" movie, this year nearly every major studio has a horse picture in its stable.

First out of the gate was United Artists with "Equs," starring Richard Burton and Peter Firth. Based on the powerful award-winning play by Peter Shaffer, "Equus" is a boy-meets-horse movie with a gory twist. This boy has the funny habit of blinding his equine companions. Hardly "My Friend Flicka."

Over at Columbia they have an entry that's been brought along as slowly and carefully as Seattle Slew. "Casey's Shadow," starring Walter Mattau and Alexis Smith, was originally scheduled for release last summer, but director Martin Ritt insisted on returning to location at Ruidoso Downs in New Mexico for a few re-takes. In keeping with Hollywood illogic, Ritt filmed the second half of the picture first and wanted to make some changes that were in keeping with the way the story evolved during earlier sequences. Now tentatively set for an Easter release, the film tells the story of horse-owner Matthau, his sons, and the quarter horse they enter in the All-American Futurity, the world's richest horse race. First place in the 20-second sprint is worth a million dollars.

Despite the delays, "Casey's Shadow" has been blessed with its own peculiar racing luck. Almost miraculously, the colt born in the foaling scene had the exact same leg markings as the horse used in the already-filmed racing sequences. "A million-to-one shot," marveled Ritt, who, like Matthau, is an off-screen racing fan.

At MGM, they're putting new life in a venerable old campaigner. "International Velvet" is the sequel to MGM's 1944 classic, "National Velvet," which starred Elizabeth Taylor as Velvet Brown, the girl who won the Grand National Steeplechase. In the new movie. Tatum O'Neal plays Velvet's niece who makes the British Equestrain Team and rides in the Olympics. Also starring are Anthony Hopkins, Christopher Plummer and Nanette Newman. Taylor tuned down the chance to recreate Velvet, obviously preferring the real horses of her Virginia farm to reel horses of Hollywood.

However, her old Velvet co-star, Mickey Rooney, is off to the races again. Rooney is starring with a black Arabian stallion named Cass Ole in hte appropriately titled "The Black Stallion." Based on the hugely successful 1941 novel by Walter Farley, written when he was only a high school student, the movie is being produced by Francis Coppola, who has a reputation of picking winners.

Coppola's biggest problem was finding the right talent to fill the role. It seems there's an old Arab legend that considers black horses bad luck and therefore only good for sacrifice. True or not, black Arabian were and far between in Egypt, Morocco, etc. It was only when the producers got back to the good ol'U.S.A. that they found Cass Ole, and eight-year-old national champion who was immediately signed for a seven-year contract (there are some 17 books in Farley's series). Included in the contract are some insurance clauses that Rooney probably missed out on: $200,000 against death, $150,000 against infertility, and $100,000 should he be unable to be shown again. To add insult to injury, Rooney has to make do with only one stunt man. Cass Ole has thre stunt horses.

After a dry stretch, it looks like horses are back in the running in Hollywood circles. As Barbara Howar said on the now-defunct "Who's Who," after watching a segment on a colonel in the Midwest trying to bring back the cavalry: "I don't think horses will come back, but then I said that about the '50s."