I see people mentioning adoptables are creations you buy. While this is very common practise, adoptables can also be given away for free. You literally 'adopt' a character.
So in plain laymen's terms, an adoptable is: A character design you wouldn't have made, but someone else made. However, you own the rights to that character.
But, if you -for example- only have artwork of that character from the person who created said character, you can only mention that the character is yours, but the artwork and design is theirs.
The character is your own, and you can use that character as you do when you make your own entirely. But the design -and any artwork- the original designer makes is not 'yours'. Just the character.
Makes sense?