I've seen the (S) in merchant and if it is smilidons I bet it is in the home nation. The (M) is in home nation burning skeletons I've had lots. And the (L) is in Apostle of Cold or Apostles of Flame I'm guessing not basic Apos.
Those (S) are only going to change into one of these 3 - Bandicoot, Kecoon, Bulcan.
Just so you know. Pet levels are the same XP as char levels and you get more choice at level 40 with (M).
Look down a few posts here for image change stuff.