Crab, Razorback 2014

A creature that touches the crab or hits it with melee attack while within 5 ft. of it takes 3 (1d6) slashing damage.

If the crab moves at least 15 ft. straight toward a creature and then hits it with Shell Bash attack on the same turn that target must make DC 13 Str save or be knocked prone. If the target is prone the crab can make one Claw attack vs. it as a bonus action.