When I was reading the Unlisted Pets section here, I looked on Wowhead to see the details.Not all who wander are lost.
I also jumped to NPC category containing Battle Pets and figured out that there are more "lost" companions!
Would you mind if I list them all here, not as a patch here, but to create a place for discussion?
Still I'll skip these that are or were in-game, but as normal NPCs or summoned by trinkets, quest items and glyphs.
Alarming Clockbot
Albino Snapjaw
Aqoo – I guess that this one was just a scrapped waterfly for Erris/Kura dailies.
Beaver Yearling – Yet another Pandaria beast.
Bitey – I guess that this one was just a scrapped plant for Erris/Kura dailies.
Blue Racer
Bubbles
Bye – I guess that this one was just a scrapped mouse for Erris/Kura dailies.
Creepy Rag Doll
Cockatoo – Just a parrot model, probably for an NPC or as a quest reward.
Corrupted Kitten – This was actually a part of an obsolete quest chain, but I wanted to show this model.
Cottontail Rabbit
Critter – I guess that this one was just a scrapped mouse for Erris/Kura dailies.
Cure for Insanity – Introduced with WoD, interesting… Perhaps a part of a quest? Good that Yogg-Saron doesn't know about this.
Dart Frog – People suggest that's [pet]Mojo[/pet].
Deep Twilight Serpent
Doom – I guess that this one was just a scrapped plant for Erris/Kura dailies.
Dr. Dragon – I guess that this one was just a scrapped whelp for Erris/Kura dailies.
Eagle Owl
Echo Batling – A bat with Spires' model? I want this one!
Elven Wisp
Essence Font
Faeling – An unused faerie dragon.
Flower Child – I guess that this one was just a scrapped podling for Erris/Kura dailies. Or a reward from these.
Frost Ravager Hatchling
Goblin Garden Deforester
Gold Ravager Hatchling
Gooey – I guess that this one was just a scrapped ooze for Erris/Kura dailies.
Grinder – I guess that this one was just a scrapped stonegrinder for Erris/Kura dailies.
Hand of Veknarius
Hawksbill Snapjaw
Island Frog
Kekek
Krik'thik Scentlayer Stalker
Leatherback Snapjaw
Leonardo – I guess that this one was just a scrapped goren for Erris/Kura dailies.
Lizzy
Loggerhead Snapjaw
Mechanoman
Mystical Experiment
Onyx Panther – This does exist, but on Asian servers only.
Olive Snapjaw
Prairie Chicken – An old version of [pet]Westfall Chicken[/pet]
Red Ravager Hatchling
Ribbon Snake
Roo – A Kekek's opponent
Rowdy Red Wind-Up Warrior
Ruby's Dance Vehicle
Rumbling Rockling
Sapphire Owl
Scarlet Snake
Seaspray Albatross – Interesting colouring.
Serpentus – I guess that this one was just a scrapped version of Bones.
Shadow Ravager Wasp
Spotted Rabbit
Swirling Stormling
Talasite Owl
Temporal Shard
Terror – I guess that this one was just a scrapped spider for Erris/Kura dailies.
Thunderhoof – I guess that this one was just a scrapped riverbeast for Erris/Kura dailies.
Tiny Green Dragon – This does exist, but on Asian servers only.
Tiny Red Dragon – This does exist, but on Asian servers only.
White Tiger Cub
Widowling DoT – A placeholder name, but I wonder if these weren't supposed to be in the place of [pet]Forest Spiderling[/pet]s in AU Shadowmoon.
XS-001 Constructor Bot
Young Frostwolf
I assume that NPC IDs can hint the former purpose of at least some of them.