I have to develop an iPad (native)app which can display SharePoint(2013 or online) forms (ASPX forms) and allow the app user to fill the forms even when the app is offline.
Based on my searches, there does does not seem to be a way to retrieve the ASPX forms from an iPad app.
There are a few apps (such as the Filamente, Formotus, harmon.ie etc.) which seem to allow that, although I have not tried.
I looked at the web services here but could not find a webservice which provides the forms (although there is a forms & a list web wervice)
Can someone let me know how I can achieve this?
You are on the good way ! Actualy there is no API to retreive sharePoint Forms (except to use IFrame but it's ugly ...)
For you (in iOS) there is only the REST/OData API of SharePoint.
You must build your own form based on the list structure (where you want to add data). To retreive all the Fields of your list or content type, you can query the FieldCollection of your list (getting fields collection). This will give you the structure of your list (fields name, type, ....), after you can build your own form with your custom logic.
you must always use REST/Odata to add/edit/delete items. here tuto from Microsoft : Working with lists and list items with REST
For the offline it's ios dev, i can't help you on this.
I hope that it will help you