Keep at it! The pet will drop eventually. Or as Venun suggested, the option to purchase or trade for it is always there too.
I had tons of trouble getting the crab to spawn as well, but rather than focusing my efforts on spawning it (which just led to disappointment), I took an alt with skinning out to the Timeless Isle and just killed crabs for leather. That felt much more rewarding than hoping for the rarespawn to pop up.
Also, concentrating on a relatively small area seemed to work much better for me than trying to kill and keep an eye on a large stretch of beach. Here's a Wowhead map
of the general area that I stuck to when killing crabs. It's next to the alliance flightpath, and there's a small cluster in the area. I managed to get 2 rare crabs to spawn there (not on the same day though). I'm sure the horde have a similar set up near their flightpath.
But it really is frustrating that the rare can spawn at any other point where a crab's been killed, so I guess you just have to hope for the best in your small section of the coastline.
Someone needs to explain to Blizz that frustration caused by such low drop rates on hourly spawned pets and even worse on mobs that hundreds must be killed to make a rare spawn, that also has a low drop rate is not what pet collectors consider fun and such things are one of the reasons they continue to loose long time subscribers. Some things meant to be fun should be just that made fun and rather easy not so time consuming of endless hours of gameplay. JMHO
Prior to the implementation of pet battles I would have argued that low drop rates and requiring to farm WAS the fun. It was the thrill of the hunt, "just one more kill..." that made it interesting. Of course, back then there weren't over 500 pets to collect though, and there weren't various options for collecting them.
These days there's a high demand for pets and more of a desire for instant gratification. There are just TOO MANY now that it would be unrealistic to keep a 100% old-school mentality of "farm for everything yourself", and that mentality is only further diminished by the increased availability of most pets.
Perhaps Blizzard needs to keep this change in mind when implementing pets, but for the most part, I think they've done a fairly good job balancing between old-school mechanics and post-pet battle collecting mentality. I mean, if they made the majority of new pets NOT cageable, then I definitely could say they need to rethink everything.