Elephants have the Trampling Charge trait (MM, p. 322):
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.
If the Elephant is actually a Moon Druid in Wild Shape, with 3 levels of Battlemaster Fighter, he can use Trip Attack to knock the target prone.
Is Trampling Charge still triggered? In other words, if the target is already prone or knocked prone through means other than the elephant's gore attack, can the elephant still make a stomp attack against the target as a bonus action?
Best Answer
The elephant can still make its bonus-action stomp attack if it meets the other requirements for Trampling Charge
In a series of unofficial tweets in December 2015, Rules designer Jeremy Crawford answered a similar line of questions about the Lion's Pounce ability, which works similarly:
For reference, the lion's Pounce trait says:
As you can see, it's nearly identical to the elephant's Trampling Charge:
Crawford's responses make it clear that whether or not the target is already prone (or, by implication, even if the target is knocked prone through some other means), the elephant could only make a bonus-action stomp attack per the Trampling Charge trait if it met the other conditions for it - namely:
If the elephant moves in that manner and hits the target with a gore attack accordingly, then it can make a bonus-action stomp attack against that same target as long as the target is prone at the time of that bonus-action attack. If these conditions are not met, no part of the trait can be applied.
A final note: The lion can choose to either make a Bite attack or a Claw attack as its action, with no targeting restrictions. However, the same is not true of the Elephant:
As you can see by the section I've bolded, the elephant can only use its Stomp attack against a prone creature. As such, if the creature is not prone to begin with, the elephant's only attack option is the Gore attack.