Games Workshop's Cash cow. A rhino is a (now implausably small for 40k) APC model, made from four plastic sprues. One sprue contains the parts to make one of the track assemblies at the side, The other makes half the central body. A rhino is made up of two of each sprue. As can be expected, both the tracks and the body are symmetrical, but they come with some bitz to disguise this (Different types of armour plate, headlights, exhausts, a cow-catcher, etc.) There is quite a bit of freedom in the posing of the design, and the hatches can be cut down the middle and stuck on in the open position.

This was the first plastic tank GW designed, and it is still in production (styrene molds last almost forever) Initially, you could pick up a box of three rhinos for a fiver. Now they're about 15 quid each. (see GW miniature prices)

Not to let a good thing go to waste, GW has made umpteen rhino variants (or Plastic Rhinos with some bits to stick on them), which go for a ridiculous markup. Initially, in the good old days, there was the predator, which came with an extended body sprue with a big gun to put on top, and some guns for the sides. They came two to a box.

Then came the grand old tradition of making the parts out of lead, and releasing a new model every time sales drop off. Such variants include:

Razorback - A rhino with less guns than a predator
Predator - A new lead predator with the gun at the front. Looks stupid. Putting the gun on the back makes it fall over.
Vindicator - Rhino with a big gun on the front.

Of course, because rhinos appeared in epic, there are other variants that GW never made a 40k-sized model for, but pretty standard conversions exist, including:
Vindicator - Much cheaper than GW's version, a rhino with a smarties tube sticking out the front.
Sabre - A rhino with a smaller gun than the vindicator sticking out of only one of the two front panels.
Tech-priest rhino - A rhino with a giant grabby arm on it. Appeared in the citadel journal, and much copied.

Apparently there is a new razorback model, but I haven't seen it yet.

2002 - There is now a new version of the rhino, more appropriately sized, and for once they've managed to modernize it without completely destroying the original design (as they did with the warlord titan, great gargant, the entire line of Daemons, and the entire Tyranid race). The new rhino looks almost identical, but slightly boxier, with more detailed moulding, and more realistic tracks. Good job!