From Destinypedia, the Destiny wiki
The High Servitors are more powerful versions of normal Servitors. They're seen to be a high value to the Fallen. Multiple High Servitor can be seen in major missions, that revolves around the Fallen. They can be big or small depending on what type it is, named servitors may belong to the High Servitors due to abilities and Durability. High Servitors can be Identifed by their yellow health bar.
Known High Servitors
List of appearances