If a monster has a skill like Skirmish or Pincher Attack, which counts as an attack, and you play Frenzy on them, would they be allowed to use the skill as second time if they haven't used both their actions for that turn?

No, Frenzy provides an Attack Action , not a Monster Action .

If a monster has a skill like Skirmish or Pincher Attack, which counts as an attack, and you play Frenzy on them, would they be allowed to use the skill as second time if they haven't used both their actions for that turn?

No, as Dam said, Frenzy only grants a regular attack action. Also, the attack granted by Frenzy is in addition to the monster's regular two actions. It doesn't matter if the monster still has his second action remaining or not.

So if as an additional question what are the hero special attacks that cost an action called? Are they just regular actions as well while an "attack action" is a normal attack?

heroes can attack for how ever many actions they have to spend (and normally that is two) unless a skill lets them do something special. If a skill reads "spend an action (or has the arrow symbol) then/and perform an attack..." then the skill requires an action to perform.

Ok so basically an attack action is strictly a standard attack. Those actions that produce special abilities through an attack are basically a normal action. Cool this was starting to come up in my group and a lot of cards use the term "attack action" which gets a little confusing. Thanks for clearing that up.