The Pentium Pro was a temporary departure from Intel's Pentium line, for several reasons. The naming was one, the "Pro" designation being an interposition into the other models, which were designated with Roman Numerals. (Although of course, since the only Pentium released at the time was the original Pentium, no one knew that "Pro" wouldn't be part of the convention.) But the most important difference was available to those who were actively working with the hardware of the Pentium-Pro: it is different and larger in form factor than other Pentium processors.
The Pentium Pro is about 2.5 inches square, larger than the processors that came before it, and about 2 ounces in weight, also somewhat heavier. Before actually measuring the dimensions of the processor, I thought it was an elongated rectangle, but that is just an optical illusion caused by the fact that its pins are arranged in a rectangle. One of the major tenets of the computer industry is Moore's Law, that more transistors can be fit in an area every year. The technical formulation is more complicated, but it basically means that things get smaller constantly. The Pentium Pro was an exception to that, being larger than its predecessors, much larger than its descendants, and with the exception of the slot based processors, was the largest standard architecture processor made. As said above, this is because the Pentium Pro included the cache onboard, so the increased size was because the "processor" included more than just the processor. Once the cache size was improved, processors continued their miniaturization.
Perhaps because it was a workaround between the Pentium and Pentium-II lines, the Pentium Pro never seems to have entered into wide currency. It was used for workstations and servers, but never had the type of marketing campaign that Intel's other lines had. However, its large shape and size still make it very desirable for two things: the recycled gold content of these processors is high, and they can also be used as gold hair combs.