Keen Hearing and Smell. The otter has advantage on Wisdom (Perception) checks that rely on hearing or smell. 

Darkvision. 60ft.

Enlarge and Reduce. The otter is able to cast the Enlarge and Reduce spell on itself, but with a special addition to the spells. It can only use the spell three times a day. However, it is able to enlarge to a maximum size of Large and can reduce up until the Tiny size. 

Enlarge. The otter's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category - from Medium to Large, for example. If there isn't enough room for the target to double its size, the creature or object attains the maximum possible size in the space available. Until the spell ends, the target also has advantage on Strength and Strength Saving Throws. 
The otter can enlarge itself to a maximum of a large size. 

Reduce. The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category - from Medium to Small, for example. Until the spell ends, the target also has disadvantage on Strength and Strength Saving Throws. 

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: (1d6 + 1) piercing damage.