In which case we can probably assume these are offensive (Chest), defensive (loins) and flanking (Horns) bonuses.
Not exactly; I'd assumed these were Zulu bonuses, but wasn't aware of the connection AriochIV had mentioned. Here's how it works:
First promotion: Buffalo chest - gives these bonuses:
<OpenAttack>10</OpenAttack>
<OpenDefense>10</OpenDefense>
<FlankAttackModifier>25</FlankAttackModifier>
<RangedDefenseMod>10</RangedDefenseMod>
Second promotion, which requires Buffalo chest: Buffalo Loins - gives these bonuses:
<OpenAttack>10</OpenAttack>
<OpenDefense>10</OpenDefense>
<RoughDefense>10</RoughDefense>
<RoughAttack>10</RoughAttack>
<FlankAttackModifier>25</FlankAttackModifier>
<RangedDefenseMod>10</RangedDefenseMod>
Third promotion is for the Impi only; an inherent trait which is not available with XP points: Buffalo horns - gives these bonuses:
<FlankAttackModifier>25</FlankAttackModifier>
<MovesChange>1</MovesChange>
<RangedDefenseMod>10</RangedDefenseMod>