As mentioned in the Introduction of this book, the main ability that the Thestral has, which separates it from its counterparts, is the ability to be invisible (at least to those who have not seen death) In other words, they are only visible to people who have seen someone dying and fully accepted, understood and internalized the concept.

Being avid hunters, and carnivorous creatures, Thestrals have an extraordinary sense of smell and will easily recognize the smell of blood and fresh flesh, even if the source of the scent is rather distanced. This ability comes innately to them. Also mentioned earlier, was their strong sense of direction. The Thestral can understand exactly where their riders need to go. If their riders have a certain destination in mind, they only need to say the destination and the creature will diligently carry them to the intended location — much like owls do with letters.

Another extremely important ability that stets them apart, is their ability to fly at incredible speeds (possibly faster than a Firebolt broom stick). Likely due to their light frame, thus ability to stay aloft with less energy. Although, the Thestrals wings do create enough list for two passengers.