What does Panther Mastery reward?

No reward has been recorded for Panther Mastery yet.

Where does Panther Mastery start?

Sir S. J. Erlgadin in Stranglethorn Vale offers it.

The quest log lists it at level 38.

No. The quest exists in Classic Era as well.

World of Warcraft: Forever · quest · level 38

A level 38 quest in World of Warcraft: Forever, offered by Sir S. J. Erlgadin in Stranglethorn Vale. Recorded by 1 player running the addon.

Pay shows once a player with the addon opens the quest.

No reward item has been recorded for this quest. The offer window fills this in when a player with the addon opens it.

What does Panther Mastery reward?: No reward has been recorded for Panther Mastery yet.
Where does Panther Mastery start?: Sir S. J. Erlgadin in Stranglethorn Vale offers it.
What level is Panther Mastery?: The quest log lists it at level 38.
Is Panther Mastery new in WoW Forever?: No. The quest exists in Classic Era as well.

What sells at this level, and what to keep from these rewards, is in the level 30 to 40 gold guide.

Source: what players running the addon saw in the quest windows; the quest text itself stays in the game. Every quest seen · Stranglethorn Vale