Basic Rules, pg. 144
Keen Hearing and Smell. The mastiff has advantage on Wisdom (Perception) checks that rely on hearing or smell. Mount Training. The mastiff is well trained for use as a mount. The rider gets advantage on any riding and other checks that use animal handling.
Bite. Melee Weapon Attack: +3 to hit, reach 1 sq., 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.
Forest, Hill, Urban