Given the Grootslang and its elephantine appearance, the ears are quite easy to explain as actually being a hood, similar to the kind seen within Cobras. Similarly, the tusk can be explained as being the creature's fangs, which do not fold back into the mouth like most venomous species'In a unique twist of evolution, the Grootslang had developed an extendable upper 'lip' which bears remarkable similarities to the trunks seen within Elephants (albeit, nowhere near as flexible). The Grootslang utilizes this lip to grab and hold onto far larger prey items than most snakes, and even provide a slightly greater reach to most of its strikes.
[Imagine, if you will, a mashup of the two following images]
In the hypothetical world of Cryptozoologicon. The Grootslang is the world's largest, and most unique, member of the Cobra family. Growing between 10 and 20 feet in length, the Grootslang dwarfs its 2nd largest relative, the King Cobra. Like the other members of the Cobra family, the Grootslang is equipped with a pair of fangs. Unlike them, however, they lack venom and are incapable of being retracted. This has left some scientists to theorize they are utilized primarily for display when attracting potential mates, or to ward off those who would approach their lair. Yet the most distinguishing feature of the Grootslang lies within its elongated upper lip, which is believed to have developed in order to provide the large, and consequently slow, animal with greater reach during its strikes. Such an adaptation allows the animal an even greater margin of error when it comes to ambushing any potential meals.
Credit of the idea goes to Dreadflyer on SpaceBattles.