| Name | Color | Sex | Year of birth | Microchip | Sire | Dam | Breed | Breeder |
|---|---|---|---|---|---|---|---|---|
| Under the Rose | br. | f | 1927 | Sand Mole | Yellow Blossom | Thoroughbred | USA | |
| Under the Table | bay | c | 1965 | Round Table | Lending Hand | Thoroughbred | USA | |
| Undercover | chestnut | f | 1999 | Allen's Prospect | Satin Lilly | Thoroughbred | USA | |
| Understanding | chestnut | c | 1963 | Promised Land | Pretty Ways | Thoroughbred | USA | |
| Understudy | chestnut | f | 1910 | Star Ruby | Bracegirdle | Thoroughbred | USA | |
| 5748-7 Undertheboardwalk | bay | f | 2011 | 250259805648577 | Green Tune | Promenade Again | Thoroughbred | Ecurie des Monceaux |
| 5352-7 Underway | d.-bay | f | 2012 | 250259806037468 | Giant's Causeway | Alchemilla | Thoroughbred | Wertheimer & Frere |
| Undeterred | d.-bay | f | 2009 | More Than Ready | Social Director | Thoroughbred | USA | |
| Undina | grey | f | 1844 | Allegro | Utka | Thoroughbred | ||
| Undina | bay | f | 1993 | 3510-1 White Reef | 5861 Surzha | Thoroughbred | APK "Kavkaz" |