Indentured Spriggan

Whenever another creature enters the battlefield under your control, flip a coin. If you win the flip, put a +1/+1 counter on it. If you lose the flip, put a -1/-1 counter on it.
"If you ask me, he's just guessing! Doesn't even find the right tooth half the time! Unless this little scamp learns his craft soon, we'll all be in dentures!"