Mastiff is a Medium beast with unaligned alignment and Challenge Rating 1/8 in the 2014 SRD. This stat block includes AC 12, 5 hit points, speeds, traits, and actions.
Bite - Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage. If the target is a creature, it must succeed on a DC 11 Strength saving throw or be knocked prone.