The second annual Middleburg National Horse Show, running Sunday through Thursday at the Foxcroft School show grounds, will feature 674 horses in 110 events, including three special hunter classics worht $1,000 each.

Total prize money for the show is $20,000. The show this year will have "country fair" overtones with clothing boutiques, fashion shows, equestrian-gear shops and elephant rides for the children.

In the $1,000 Middleburg National Hunter Classic, Lee's Leiutenant, owned by Mr. and Mrs. W. T. Cook and ridden by Bobbie Stedding, is a favorite. Others entered in the classic include Ralph The Roman, ridden by Kathy Doyle, and Bold Acquisition and Chadron, both ridden by Don Stewart.

In the $1,000 Junior Amateur Owner-Hunter Classic, Louise Lefferts' Little Elyer is the defending champion. The horse was entered in last year's classic under the name of Snoopy. Leffers also has entered her horse Cross roads in the competition.

In the $1,000 Middleburg National Pony Hunter Classic, Lefferts will ride Irish Brass, last year's large pony champion. Mildred, owned by Mrs. Jordan Bentley and ridden by her daughter, Leigh, also is entered in the pony classic. Mildred was last year's large green pony champion.

Lisa Capper from Fairfax County will ride White Oak in the pony classic.

Dave Kelly of Southern Pines, N.C., a nationally known rider, will guide Royal Reveler and Sir Thomas in the regular conformation hunter division.

In addition to elephant rides ($1.50) this year, there will be a llama and a goat for children to pet. Admission is $1 for adults and 50 cents for children under 12. The competition Sunday begins at 8:30 a.m.

The Foxcroft School show grounds in Middleburg can be reached by taking Rte. 626 off Rte. 50.