Bullrat

Strong like bull: The Bullrat has advantage on grapple checks, strength checks, and strength saving throws

Swarm like rat: If one or more Bullrats is within 40 ft. of this Bullrat, the Bullrats can take the following action: All Bullrats within 40 ft. of this Bullrat, including this Bullrat attempt to tackle a creature. All Bullrats participating in this action swarm to the creature this Bullrat is attempting to tackle. This Bullrat makes the athletics check, and each other Bullrat adds +2 per Bullrat to the total. if the creature fails the check, it is grappled and knocked prone by all of the participating Bullrats. The creature escapes the grapple if it escapes each individual Bullrat's grapple, move all of the grappling Bullrats off of itself, or all the bullrats grappling it die. All participating Bullrats sacrifice all of their next actions in order to partake in this action.

Nimble: The Bullrat can be in another creatures space.

Gore: Melee weapon attack, +9 to hit: 1d8 + 5 bludgeoning damage and 1d6 + 2 piercing damage.

Tainted bite: Melee weapon attack, +6 to hit: 1d4 + 2 piercing damage, and the target must make a constitution saving throw with a DC equal to 10 + the damage dealt. On a failed save, the creature takes an additional 1d4 poison damage and contracts sewer plague.