I downloaded Echo’s rig from the slay project and it comes with her normal hair (bun) and there’s a ponytail skeletal mesh as well. I want to add the pony tail and simulate it but I can’t figure it out
Hello, Welcome to the Forums.
The Game Animation Sample has Pony-tail Echo as a selectable character, so seeing how they are setting things up might give a hint. (Just step on button to show selection window)
The bones in your image reminded me of spider legs - I prompted ChatGPT: “Can you make the white bones lines into a spider with a pony tail?” 
It needs the ponytail ABP and physics asset to simulate.
The Echo sample and GASP have it working. Maybe Slay left it out? The Echo sample had a bool variable to toggle visibility between the groom bun and the ponytail, so Slay might? Echo’s groom stopped working in 5.6 last I saw.
@Astrotronic That’s awesome and terrifying lol