hey hey 
There is a solution for your problem, but it’s a bit technical. If that doesn’t scare you off, give this a try.
Your pet names are stored in your character preferences. If you remove them from there, they revert to the default.
Step 1.
First let’s find your character id. In game, press SHIFT-F9. It will show the following information:
Extended location information:
-204.0, 211.0, 17.7 (204.0 211.0 y 17.7 500)
-Pf Proxy: Model=51100:500 GS=0 SG=0 R=500, resource: 500
-zone: 80, area: “Parnassos”
Server id: 3136, character id: 50000:158027, time: 2010-02-23 19:03:50 (UTC)
Version: 18.3.1_EP1, map id: 18.3.1.4_10.12.2009_13.16.26, build: 48468.
You can see the bit “character id: 50000:158027”, which means 158027 is the id we’re looking for. It will be a different one for you.
Step 2
Second, locate your preferences folder. Close the game and start the launcher. In the launcher, click the settings button. Go to the GUI tab and click the Open button for the screenshots folder. This opens a folder in the windows explorer. Go to the parent directory and then open the directory named Prefs from there.
Open the directory with your account name. In there, open the directory named Char158027. Replace 158027 with the character id we found earlier.
Step 3
Your pet names are stored in the file Prefs.xml, which we’re going to edit. At this point I would recommend making a backup of Prefs.xml.
Right click Prefs.xml, select “Open with…”, “Choose another app”, and pick Notepad from the list.
With the file open in Notepad, press CTRL-F to bring up the search window and search for PetName1. You should see something like the following:
<Value name="PetName1" value='"My awesome attack pet"' />
<Value name="PetName2" value='"My amazing heal pet"' />
<Value name="PetName3" value='"My incredible mezz pet"' />
<Value name="PetName4" value='""' />
<Value name="PetName5" value='""' />
<Value name="PetName6" value='""' />
Replace these lines like so:
<Value name="PetName1" value='""' />
<Value name="PetName2" value='""' />
<Value name="PetName3" value='""' />
<Value name="PetName4" value='""' />
<Value name="PetName5" value='""' />
<Value name="PetName6" value='""' />
Press CTRL-S to save and then close Notepad.
Step 4
Restart your game and verify your pets have their default name again.
If something went wrong and you accidentally messed up your preferences, restore the backup from step 3. Please feel free to ask here if you would like some more details.