Closing this as PetJournal was already added to Wratch Classic in patch 3.4.3 (commit c7e0ddb), but forgot to close this issue back then.

This should be already supported, but for some reason it isn't working?

Need also bit more information either in written or screenshot format:

• Is this about WoW Classic (aka Vanilla) or Wrath Classic?
• Are you talking about pets in Guild Bank, sold by vendors or AH listings?

Here's a screenshot of a pet vendor window from Wrath Classic for reference.

And just to rule out Elv UI. (=

Works fine for Recipes.

Here's a mount vendor window. It does work correctly for mounts! (=

Seems like Wrath either returns pet collection data differently or uses different functions for it from Retail. I'll look into the pet situation when I have time to work on addons.

If it isn't too much asked, could you go back to the pet vendor and hover your mouse over one of the pets you already own in the vendor frame and use command /ak itemtest or /alreadyknown itemtest and paste the results here for reference data.

ItemTest: [Black Kingsnake] - Regions: 18/76 - Known: false
ItemLink: |cffffffff|Hitem:10360::::::::80:::::::::|h[Black Kingsnake]|h|r
3 FontString GameTooltipTextLeft1 Black Kingsnake
5 FontString GameTooltipTextLeft2 Use: Teaches you how to summon this companion.
9 FontString GameTooltipTextLeft4 TSM General Info
11 FontString GameTooltipTextLeft5 Vendor Buy Price
12 FontString GameTooltipTextRight5 42s 50c
13 FontString GameTooltipTextLeft6 Vendor Sell Price
14 FontString GameTooltipTextRight6 12s 50c
15 FontString GameTooltipTextLeft7 TSM AuctionDB
16 FontString GameTooltipTextRight7 5 auctions (10 Hr 40 Min ago)
17 FontString GameTooltipTextLeft8 Min Buyout
18 FontString GameTooltipTextRight8 2g 84s 90c
29 FontString GameTooltipTextLeft9 Market Value
30 FontString GameTooltipTextRight9 [35%] 4g 36s 12c
31 FontString GameTooltipTextLeft10 Region Market Value Avg
32 FontString GameTooltipTextRight10 16g 82s 08c
33 FontString GameTooltipTextLeft11 ATT > Zone > Orgrimmar
34 FontString GameTooltipTextRight11 Vendor > Xan'tish
37 FontString GameTooltipTextLeft13 ID 10360

Pushed out version 1.53 with some kind of bandaid fix for this, but beware since I strongly assume there will be some false positives/negatives with it. As far as I know, there isn't any real API to check if the companion pet is already known or if the vendor item has any relation to any companion pets you already own.

I spent way too much time last night trying to find a way to pull the pet names... yeah... I just assumed there was a call for that. Tough spot for something so trivial. Thanks mate! I'm sure whatever you got done is good enough! (=

The current method compares the icons and sees if there is at least a partial match between the item name and the companion pet name so it should work for most of the companions, but might miss some of them.

And btw. if you are interested in finding out the pet names from your collection you can use this kind of approach:

for i = 1, GetNumCompanions("CRITTER") do
	local creatureId, creatureName, creatureSpellId, icon, issummoned, mountType = GetCompanionInfo("CRITTER", i)
end

but without making some kind of translation table between vendor sold itemId and collection creatureId there isn't 100% sure way to match these.