OK I don't believe I have seen this question before - When I double click the packaged sims 2 pet that I downloaded from Sims 2 Resource, this is what appears: "This package requires Expansion Pack data this is not installed". Now how can this be? I have installed Sims 2 Pets and I have been playing this pets that I have made and even ones that I have downloaded from other sites. But these ones I get this! What do I do?

You need to open it with Pets Bodyshop installer, and it seems as though your game is defaulting to an older version. I found this very problematic in my game- but what you need to do is to right click on a sims2pack, choose open with> choose program and browse to Program files/ EA Games/ The Sims2Pets/ CS Bin/ Package Installer.exe. Hopefully that will make all sims2packs open with the pets installer. However, my computer still refused to do this, so I ended up just dragging the sims2pack I wanted to install onto the Program files/ EA Games/ The Sims2Pets/ CS Bin/ Package Installer.exe icon, and that did the trick. Alternatively, try the Clean Installer.

http://www.modthesims2.com/article.php?t=86150

Ah, I didn't know about that article- however it is incorrect in saying the only workaround is to use the CleanInstaller; dragging the file to the Package installer icon definitely works. I had the Lycos Dog from one of the European websites, and for some reason the dog just would not install correctly with the Clean Installer. I had to use the Pets Bodyshop Installer.

Thank you very much. My computer is doing this and it appears to be working. I will let you know once I get into the game. Thank you again.

Okay, didn't know it was incorrect. I stumbled across that article a couple of days ago.

I don't suppose you still have the download? I've been searching for it, and every place I've found is either dead or "you must register, but there's nobody home to accept your registration." Thank you so much.

^ Highly doubt it, since the topic is nearly 18 years old, and the OP hasn't been active here in at least 4-5 years.

Try this one:
https://simfileshare.net/download/5068585/

Thank you! God Bless Sims CC Hoarders!!

It's only the collar unfortunately, there was a dog (which just looked like a standard black lab) and a sim with house too, but I don't seem to have kept those.