So, what about a monster with tentacles? They can potentially lift a lot.
In running my games I would probably use the same lifting/carrying rules as for any other appendage. As the tiefling is coming home from grocery shopping they could fish their keys out of their pocket with their tail but couldn't use their tail to unlock the door. They could then set one back down while using their hand to unlock and open the door. As they walked in, I would let them pick up the back with their tail and carry it in.
I'd say a STR 18 character can lift a lot more with their tail than a STR 8 character.
As a side note, a player wanted to use Sleight of Hand to pick the pocket of an NPC. She wanted to use her tail to do it. In game mechanics, it made no difference. In RPG style it was pretty cool and made the session more fun. I went with it but explained that other PFS GM's might not allow it.
Don't forget, we are all here to have fun.