This might be radical but I call them...supplements. Even worse, I have done for the last 40 years, because the idea of releasing supplemental stuff isn't new for D&D, or any other RPG. I have bookshelves filled with supplements to prove that point.
On the vaguely ridiculous idea that no one ever had apprentices, squires or raised monsters. I have had one character, a very LN fighter, who after accepting the surrender of the goblin, decided to train him as his squire. Because he was obviously a goblin, he was made to wear a sack over his...