I know that the word bastard in this sense appeard only in 13th century. So what was the normal term before that?
Actually, in the Old English days, the word for “bastard” was cifesboren.
There are many references in genealogies and histories of natural sons and natural daughters of nobles and royalty during the Middle Ages -- people like Meiler Fitzhenry (son of Henry I of England), William Longsword (son of Henry II of England), Hamelin Plantagenet (son of Geoffrey of Anjou), and Joan of Wales (daughter of King John of England). These references date from the 12th century, and the same terms appear to have been in use as late as the 18th century.
In Medieval Latin, dating from the 11th Century, the equivalent was bastardus.