I just used a toy to get the achievement and it disappeared. No other toys I've ever used have disappeared when I used them to play with a pet. Is this something that happens because of the achievement or is it a glitch?

It's not a glitch, toys are supposed the dissapear on subeta if you play with them :) If it didn't dissapear before, it's because your pet was too happy and didn't actually play with it.

Seriously? Somehow I didn't know that. Well that blows :( I won't have used that toy for the achievement. Thanks.