If a 500kg horse is moving at a speed of 15 mph what is its kinetic magnitude?
You probably mean its "kinetic energy".
To get it in Joules, use the formula
Kinetic Energy = (1/2) M V^2,
with the speed in V in meters per second. Mass M must be in kg, so use 500 kg for that.
15 mph = 22 ft/s = 6.706 m/s
K.E. = (1/2)(500)(6.706)^2 = 11,243 J
That should be rounded off to about 11,200 J due to the low precision of the speed number.