Ivorytusk Fortress tells you to untap your creatures with +1/+1 counters during your opponent's untap step, while Icy Blast and its kin say they don't untap during their *controller's* next untap step. This means there's no problem untapping during the opponent's turn!
Last and certainly not least of the five clans, we have the Temur Frontier. They live in the coldest parts of Tarkir, and revere the savagery of the dragons. Their symbol is a set of dragon's claws, and their mechanic certainly feels savage! Ferocious is another ability word, like Raid from earlier this week. It has no rules meaning itself, and if you took "Ferocious" off of all their cards, those cards would still work. The ability word is just there to group thematically similar abilities together