There Are Two Abilities
Among the books and resources I have searchable access to, I find only Prestige Class features, no feats (at least none that have Mounted Combat as a prerequisite, or that mention it or the word 'negate' in the description)
Here are the prestige class features:
Aglarondan Griffonrider (Unapproachable East, p18) gets
Improved Mounted Combat (Ex): At 3rd, 5th, and 7th level, an Aglarondan griffonrider increases by one the number of Ride checks she may attempt each round to negate a hit against her mount (two times per round at 3rd, three times per round at 5th, and four times per round at 7th). The hit is negated if the griffonrider's Ride check result is greater than the attack roll.
Ashworm Dragoon (Sandstorm, p66) gets
Relentless Shield (Ex): At 4th level and higher, you can make a Ride check after any attack that successfully strikes your ashworm mount, in an attempt to negate the hit on your ashworm. Normally, the Mounted Combat feat allows you to make this attempt only once per round. See the Mounted Combat feat, page 98 of the Player's Handbook, for details. In addition, you gain a +2 bonus on Ride checks made to negate an opponent's attack roll. This ability does not extend to any other type of mount.