DiabloTX, and for the uninitiated, that 55 Chevy in the photo is NOT a "muscle car".
The phrase "muscle car" has its roots in the early 60's, when Pontiac dropped a big block engine into, what was then, a small car. Thus any smallish car, with a big block motor is a "muscle car".
"Pony car" is your basic 2 door hardtop, which only has a backseat to fill the gap between the back of the front seats and the trunk. Typically the back seat is useless, unless it can be folded down. Horsepower is a bit irrelevant in this definition. This phrase came about more for the need to differentiate from "sports cars". In Europe, the auto purist would come unglued if he had to refer to a Mustang or Camaro as a "sports car".
"Hot rod", in the traditional sense, is an open wheel roadster, but has been used since those days to describe a car which has been modified from stock to include, but not neccessarily require, more horspower and/or better suspension.
All the above are designators for street cars. None apply to race cars. In this context, "race cars" are cars which cannot be legally driven on the street. All the above can be legally driven on the street.
Now Diablo, the 55 Chevy in your photo, is a hot rod. It's not a small car, scratch "muscle car". It has a practical back seat, scratch "pony car". It certainly is not a "sports car". Thus, it is a "hot rod".
"Street Rod", is one of the newer terms (all things being relative) in hot rodding and it really is the same as "hot rod", but considered a sub-category of "hot rod", for those who get confused about street legal cars versus race cars, or near-race cars.