Field-Born Nibblers

Whenever a creature you control with power
6 or greater attacks, put a +1/+1 counter on
this creature.

As long as this creature has three or more
+1/+1 counters in it, it has trample.

As long as this creature has six or more +1/+1 counters on it, it has menace and "This creature can't be blocked by creatures with power 2 or less."