Scraf Mouse is a new card from the Big update (Beta 6.6).
It's a cheaper version of Nacarat Jester, but is only able to silence the monster in front of it. This makes it less effective as it can't cure a monster affected by cards like Toriel. However, it can still be useful due to its low price, making it good for a Bravery Deck.
Since 26.0, it can now be found in all beginner's card pools.