Friday Fun: Riddle - What Am I?

Lol its friday lets have fun... Below is a riddle hope you can guess it!

I can be repeated,
But often not in the same way.
I can't be changed,
But can be rewritten.
I can be forgotten,
And can also be lost with death.
My first is in horses,
But not in ponies.
My last is in pretty,
But not in beautiful.

What am I?