The short answer is that they measure something completely different.
It is not really possible to directly compare thrust to horsepower. The thrust numbers quoted by engine manufacturers are usually for static thrust at takeoff. Horsepower is defined as the ability to do 33,000 foot-pounds of work in one minute. The problem is thrust is a static number, if the engine is bolted into a test stand and is roaring away making 115,000 lbs of thrust (I believe this is what the newest 777 engines are rated at), no movement is occurring and the engine is creating zero horsepower.
Since horsepower is a measure of the ability to do work (work = force x distance) then some movement must occur.
If you want a quick answer:
One HP = 33,000 foot-lbs in one minute, so...
33,000 foot-lbs / 5280 (how many feet in a statute mile) = 6.25
6.25 x 60 (60 minutes = 1 hr) = 375
So, the short answer is that at 375 mph, 1 lb of thrust = 1 hp
If you want to use nautical miles instead, divide 33,000 by 6000.
Now the problem is, how much thrust is an engine making at speed? The thrust of a propeller goes down with an increase in speed, but the thrust of a turbojet usually increases due to ram recovery (the faster the aircraft is going, the more air is getting shoved into the front of the engine). On a large turbofan, I am not sure of the final outcome, since most of the thrust is generated by the fan (which is really just a shrouded fixed pitch propeller).
