Not sure if these would be the "hardest" but if you come across any of the wild hippos or the Shadow Sporebat
, I'd capture 'em (regardless of quality). Those seem to be sparse compared to other wild pets in Draenor. Ironclaw Scuttler
might also be a contender for 'capture on sight' heh.
The majority of the other pets just require patience and dedication. There's a lot of grinding rep, collecting currency, etc.
To me, the hardest WoD pets to collect are the ones that rely more on RNG. The pets that drop from herbalism and mining for example. Or the one that you can randomly create while crafting Draenor food. I never managed to get those to actually drop for me while on the beta, so I expect it to be just as difficult on live. x_x