ADD_ATTRIBUTE_POINTS

C# example (by KeyValue)

Horse Rank
attributeIndex = 7

Rank 1 = 50
Rank 2 = 250
Rank 3 = 550
Rank 4 = 950

Use SetAttributePoints on horse ped
Each level will increase both health and stamina

Example:

AddAttributePoints(HorsePed.Handle, 7, points);