The notation of increased CMD for having more legs is a courtesy, the rules still apply even if they're not indicated on the statblock. This is because the increased CMD is a rule before it's applied to creatures:
Trip itself indicates
If the target has more than two legs, add +2 to the DC of the combat maneuver attack roll for each additional leg it has.
Since this is a blanket statement, the Animal Companion section could indicate that they don't receive this general bonus (causing a specific trumps general situation) but it doesn't, it just gives you a base stat block.
The riding ant having it noted is a symptom of being from a Third Party Publisher (3pp). 3pp write their own material and entries that usually conform to how Paizo phrases and structures their material, but not always. It was simply a matter of Jon Brazer Enterprises including what Paizo implied.
As to racial abilities and skills, if it's not indicated in the stat block, they don't have it and use the standard 'class' skills that are on the Animal Companion information. The exception to this if if they have a Climb or Swim speed, they still retain the +8 to that check from having the speed, for the same reason as CMD above.