Keen Hearing and Smell. The mastiff has advantage on Wisdom (Perception) checks that rely on hearing or smell.
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. Mastiffs are impressive hounds prized by humanoids for their loyalty and keen senses. Mastiffs can be trained as guard dogs, hunting dogs, and war dogs. Halflings and other Small humanoids ride them as mounts.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.