Each creature you control with a +1/+1 counter on it untaps at the same time as the active player’s permanents. You can’t choose to not untap them at that time.
20-09-2014
wotc
Effects that state a creature you control doesn’t untap during your untap step won’t apply during another player’s untap step.
20-09-2014
wotc
Controlling more than one Ivorytusk Fortress doesn’t allow you to untap any creature more than once during a single untap step.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.