Rock is a very strong Taunt card with 3/6, just like Jerry, but with +1G. And instead of passively having a potential +1 ATK, Rock has a Dust effect that'll replace it with a Pebble, which is a 1/3 monster for 1G that also has Taunt. (Its Cost doesn't matter much due to being directly summoned once Rock dies.)
This essentially means that you are protected from at least two monster attacks, if spells and effects aren't a factor. However, this does not mean that you should replace Jerry as he is cheaper and enough gold can be earned for him on the start of your second turn, whereas the Rock will require saving up for at least the third turn.
It is also recommended to save Rock for moments where two Taunts are needed/there are multiple enemy monsters on the field (typically after turn 7). Because of this, Jerry is possibly suited towards quicker Decks and the early to mid game unlike Rock. Rock is also typically a guarantee of having a Taunt on the field after a board wipe like Mettaton NEO or Hyper Goner is activated.
Keep in mind that the Pebble cannot attack right away if you kill your Rock during your turn. It can be used right away if the opponent kills the Rock, but not the Pebble.
- Just a Rock (by Diamaincrah)
- Artistical Discomfort (sold seperately)