I've never used PS before- I use gimp- but I've heard they are very similar! There's two ways (that I know of) to lengthen manes/tails.
Use the clone tool. Clone the mane/tail where the color will be similar to what you want to add and then just add the cloned part the end where you want ti lengthened. Then you have to just go through with the blend tool and make it look smoother and more realistic.
The other way is to use the color picker tool and paint it the mane. You're supposed to use multiple layers to add in shine and highlights and all those fancy things, but I'm very simple so I just draw it in however :) Then you blend a little and (hopefully) it looks good!
Again, these are for gimp, so I don't know if all the tools and such are the same on PS. I attatched my first attempt at lengthening the mane and tail. I didn't really go for realistic (obviously
) but at least its a general idea of how it looks. (I also blended the whole picture and removed the halter, very badly, so ignore that) Good luck!