Pyramid does not include a form library because there are several good ones on PyPI, but none that is obviously better than the others.
You can use WebHelpers and FormEncode in Pyramid just like in Pylons. Use pyramid_simpleform to organize your view code. (This replaces Pylons' @validate decorator, which has no equivalent in Pyramid.) FormEncode's documentation is a bit obtuse and sparse, but it's so widely flexible that you can do things in FormEncode that you can't in other libraries, and you can also use it for non-HTML validation; e.g., to validate the settings in the INI file.
Some Pyramid users have had luck with WTForms, Formish, ToscaWidgets, etc.
There are also form packages tied to database records, most notably FormAlchemy. These will publish a form to add/modify/delete records of a certain ORM class.