Aid is a character stat that has made appearances in many games throughout the series, although it has gone by many different names and formulas. Essentially, a unit's Aid stat is the maximum constitution or build that another unit can have for the first one to be able to Rescue them. The highest Aid stats are usually attributed to mounted units, such as cavaliers and wyvern riders.
In most infantry units, the aid stat is one less than the unit's constitution. In mounted units, however, it is a set value- often 25 minus the character's constitution for male units and 20 minus the constitution for females.
In the GBA games, Aid for mounted player units is set at (25-Con) for male units and (20-Con) for female units. However, all generic mounted enemies have an Aid of (25-Con), including the presumably female Pegasus Knights and Falconknights, suggesting that these units may actually be male, or an oversight on the developers part.