Taming a hippogryph presents a challenge that many Bretonnian Lords cannot turn down, and this has made them into much sought after steeds. To successfully do so, the animal must be captured and broken at a very young age but, given the relentless territorialism of adult hippogryphs, acquiring a chick or an egg can prove a dangerous proposition indeed. Occasionally, the task of appropriating such a beast is given to a Knight Errant as a way of proving himself. More often, a Duke will find suitable 'volunteers' from among the peasantry, promising a huge reward to any survivors that return with a healthy, young hippogryph.
↑Armour-Piercing: The damage of armour-piercing weapons mostly ignores the armour of the target, making them the ideal choice against heavily-armoured enemies. They are often heavier and attack at a slower rate though, making them less efficient against poorly-armoured targets.
↑Causes Terror: This unit can cause terror, making its melee target rout for a short time. Units that cause terror are immune to terror and fear themselves.
Compared to the other flying cavalry options of Bretonnia, these are their most expensive versions. They lose models, going from 18 to 6. They gain a slight boost in melee stats, making them the best melee fighters just looking at the stats. They lose a shield, but gain the ability to cause terror and fear which is very useful on low leadership factions. Finally, they lack the bonus vs. large of the Pegasus Knights but have Armour-Piercing damage instead.
Therefore these units are a bit more of a generalist compared to the ability of the Pegasus Knights to hunt down large units. Fear and terror make the hippogryphs more suited for dispersing groups of infantry and then flying out before they get caught. They also synergize best with the Lore of Life as they have fewer models (when a model dies it can't receive healing, but these ones have higher health per model, meaning they are more likely to benefit from healing).
One could also use lore of heavens to debuff enemy flying units when going up against factions like Dark Elves, High Elves, or the Empire, but it is unlikely this units will need the help.