How do I adopt stray dogs/cats/horses on the sims 3 pets?
It’s not easy. You have to befriend any stray that comes into your yard. They mostly come in the evening and they only stay a short while so getting to friend status can take time and patience. Once you are friends with a stray, you should have an interaction option to “adopt stray”.
Many people find this tedious and time consuming and use either cheats or mods to help this process along. Using testingcheatsenabled true you can drag the relationship of the stray up to friend easily. Or you can use a mod like this one here at mod the sims, that changes the amount of strays you get and how long they stay.