With Patch 10.1.5, Warlocks are receiving new pet customizations, such as new Imp and Felhunter customizations. However, the new customizations do imply in some changes for existing Warlock pets, and one of them is the unfortunate removal of the Glyph of the Observer in Patch 10.1.5, which transforms your Felhunter pet into an Observer.
Blizzard's justification for the removal of the Observer customization is that they want to keep Warlock pet appearances consistent, so that each demon can be easily identifiable by other players, as well as the Warlock. We will miss you, Observer!
WARLOCK Glyph of the Observer is being deprecated and will no longer be an available customization option for felhunters.Developers’ note: With the arrival of the new warlock pet customization system coming in Fractures in Time, we will deprecate the Glyph of the Observer customization.Our reason for this is to ensure that each demon retains a similar silhouette across all available customizations, so that each demon and the functionality or power they provide to the warlock is easily understood by other players as well as the warlock. Read more on wowhead.com