Rap Dog is a weapon in HoloCure: Save the Fans!. It can be obtained through a collab between X-Potato and Idol Song. The weapon rapidly throws large potatoes that bounce around the screen, debuffing all enemies hit and making them take 20% increased damage from all sources.
Note, that while "+1 Projectile" is a possible enchantment for Rap Dog, it has no effect on the weapon whatsoever, as it is not a Multishot type weapon.
This item is likely named after Korone doing an impressive rap showing while singing "if..." by DA PUMP with Okayu in a karaoke collab.[1] She's also improvised a rap verse during one of her other streams after expressing a desire to collab with Calliope.[2] Its mechanics, namely the added DEF debuff, resemble Calliope's similarly themed ability The Rapper.