No, the Giant Elk is given only one Action (and no Bonus Actions), and thus can only choose to do one of the available actions.
The text for Charge reads (MM, pp325)
If the elk moves at least 20 feet straight towards a target and then hits it with a ram attack...
Your Actions(and specifically those for Attacking) are as follows (MM, pp325)
- Ram
- Hooves
The Charge means you are choosing the Ram Attack.
The next round, if the creature is still prone, you may attack with the Hooves. Otherwise, it's just another Ram attack without the charge (unless you take the OA to move away and run again at them.)
And as Andras says, there ARE beasts like the Elephant (MM, pp322), that have viable Bonus Actions available to them.
Trampling Charge. If the elephant moves at least 20 feet straight toward a creature and then hits it with a gore attack on the same turn, that target must succeed on a DC 12 Strength saving throw or be knocked prone. If the target is prone, the elephant can make one stomp attack against it as a bonus action.
The (Giant) Elk, however, is not one of them.
It stacks
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
If you take the attack action, you attack twice.
At the start of your first turn of each combat, you walking speed increases by 10 feet, which lasts until the end of that turn. If you take the Attack action on that turn, you can make one additional weapon attack as part of that action. If that attack hits, the target takes an extra 1d8 damage of that weapon's damage type.
If you take the attack action, you can make one extra attack.
Nothing in these two statements prevents the other. One changes your number of attacks to a flat number (2), the other says "whatever it was before, +1".
2+1 = 3, you get three attacks in the first round of combat, and one of those 3 attacks deals extra damage. You get only 2 attacks for the rest of the combat.
On multiattack:
Multiattack is a monster specific action. It is NOT the 'Attack action', it's a specific action some monsters have, that determines what it can do with those attacks. This is to prevent, for example, a bandit captain from using its multiattack to make three grapple attempts in a single round. Both your features require the 'Attack action'.
Best Answer
Yes. Dive Attack is an ability that triggers when it says it does; it doesn't alter what the peryton is allowed to do. Since diving and then using Multiattack satisfies that condition (assuming at least one of the attacks hits), yes, it applies to whichever of the Gore or Talons attack triggers the Dive Attack ability's conditions. The peryton can then make its second attack before swooping past (or landing on the target, or whatever the plan is), since the Dive Attack doesn't prevent the second attack.