I took out wfsanity and sims rolled a lot of "get pet" wants.  I like the idea, so I've locked them on at least 3 lots.  Now I discover that adopting a stray doesn't satisfy the wants.  Makes me want to strangle whoever wrote that code.  I found an old thread so it's a known problem.  That thread pointed out that moving in a CAS made pet doesn't work either.  So, being suspicious, I thought I'd ask "what does work?" before using my playables with pet LTWs to sell/give them pets.  1 and 2 obviously, but how about the others?
1. adopt a pet from the pet adoption service by phone
2. take the sim shopping to a community lot pet shop
3. be given a pet by another sim
4. buy a pet off another sim
5. take the sim shopping to an owned business
6. come home with a pet bought on a community lot while you were not controlling them  
7. come home with a pet sold to you on an OFB business lot while you were not controlling them 
8. have a puppy/kitten born on the lot
any other options?
I tried satisfying the want by ctrl-clicking on it, but it refused to roll. wtf?  ctrl-click worked on the other 3 wants, but that one just wouldn't roll.  It's locked, but I think I unlocked it and tried too.  It was very bizarre!..... and very annoying!  
The old thread includes this post -
In my experience adopting a stray also won't satisfy the "get a pet" want.  Adopting a pet by phone or buying one at a pet store are the only ways I have seen that want satisfied in my game (I haven't tried fufilling it by having my pets have puppies or kittens yet).  I'm really disappointed that the adopting a stray interaction isn't counted because it takes more work than just buying a pet and sometimes my sims don't have enough money for a pet, but still get the want.
My elderly sim is broke too & adopting a stray was perfect...