There are two items that are using the Poisonous Potato picture.
The first is the normal Poisonous Potato. The second is labeled Regular Potato and is written in italics.
I got the Poisonous Potato from my potato garden. I think I got the Regular Potato from a treasure chest. If not it was from a zombie.
I only have one of each, so I haven't used them yet to see what happens. But I can if you need me to.
The potato named "Regular Potato" has just been renamed by the mapmaker before being put in a treasure chest. If you encounter this item in a naturally generated map, I'll reopen this issue.