Monsters have attacks listed among their actions. From the SRD,
The most common actions a monster will take in combat are melee and ranged attacks. […] Any damage inflicted or other effects that occur as a result of an attack that hits a target are described after the "Hit" notation.
So, the bite of a giant frog is clearly an attack:
Bite. Melee weapon attack: +3 to hit, reach 5 feet, one target. Hit: 4 (1d6 + 1) punch damage, and the target is trapped.
But what about other skills, or skills that require an attack? The giant frog can also swallow:
Swallow. The frog makes a bite attack against a small or smaller target that is struggling. If the attack hits, the target is swallowed and the staple ends.
While an octopus can use a cloud of ink:
Cloud of ink (recharges after a short or long break). An ink cloud of 5 feet radius extends around the octopus if it is underwater.
As a Beastmaster, can I order my teammates to use any of these skills with the Attack action? The ink cloud seems to fall into a special skill category, and Swallow has the property of "melee weapon attack", but it is essentially a bite attack (with a hook target).