Can Feasting Troll King be countered when it returns to the battlefield?
No. That's an activated ability and not a spell, and Essence Scatter can only counter spells. You'll need something like Trickbind to counter it.
Rules 112 and 113 define what spells and abilities are, and they are distinct:

112.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”


113.1. An ability can be one of three things:
113.1a An ability can be a characteristic an object has that lets it affect the game. An object’s abilities are defined by its rules text or by the effect that created it. Abilities can also be granted to objects by rules or effects. (Effects that grant abilities usually use the words “has,” “have,” “gains,” or “gain.”) Abilities generate effects. (See rule 609, “Effects.”)
113.1b An ability can be something that a player has that changes how the game affects the player. A player normally has no abilities unless granted to that player by effects.
113.1c An ability can be an activated or triggered ability on the stack. This kind of ability is an object. (See section 6, “Spells, Abilities, and Effects.”)