In the past, players might occasionally have to sacrifice a soulbound vanity pet or two due to lack of bag space. With Blizzard's new companion summoning system, we'll never have to worry about saying goodbye to our dear pets again. But even better, players who abandoned some of their former companions can visit any stable master to "relearn" certain summon spells.
I did a little testing with this feature. First I destroyed my Cat Carrier (Siamese) and a few other companions I obtained from vendors. There was no option available to relearn any of them. Next I deleted my Panda Collar and a few other blue items. I got the usual dialogue box where I had to enter the word "delete" to confirm my decision. Unfortunately, I could not reclaim these pets either.
UPDATE: Based on my testing along with what I've heard from others, there are only four pets that can be retrieved when lost (all are quest rewards):