Ok so I have noticed lot of wild battle pets not spawning properly in the pre-patch, and really hoping that it is going to get fixed soon as it makes collecting them really annoying, in the beta this was not an issue. This is probably not exhaustive list, but ones I have encountered so far that either don’t respawn at all or extremely slowly include:
Tiny Bog Beast (Wetlands)
Tiny Twister (Arathi Highlands)
Lofty Libram (Hillsbrad Foothills)
Jade Oozeling (Hinterlands)
All the various mechanical pets around Everlook (Winterspring)
Sporeling Sprout (Zangarmarsh)
Warpstalker Hatchling (Terokkar Forest)
Cogblade Raptor (Blade’s Edge Mountains)
Fel Flame (Shadowmoon Valley)
Oily Slimeling (Borean Tundra)
Nexus Whelpling (Borean Tundra)
Stunted Shardhorn (Sholazar Basin)
Emerald Proto Whelp (Sholazar Basin)
Scourged Whelpling (Icecrown)
In addition, Restless Shadeling which is supposed to spawn outside Karazhan during mornings is not spawning at all, and Naxxramas bosses do not seem to be dropping the battle pets they are supposed to.