Otter Patronus

Whenever enchanted creature attacks, put a
+1/+1 counter on enchanted creature. Then if it
has three or more +1/+1 counters on it, sacrifice
Otter Patronus.
When enchanted creature becomes the target of a
spell or ability your opponent controls, sacrifice
Otter Patronus.
Whenever Otter Patronus is sacrificed, counter
target spell or ability.