Prolog has an ISO standard... I am not sure if it's still evolving, but specific Prolog implementations can and often do add their own non-standard extensions. For example, SWI-Prolog added dictionaries and a non-standard (but very useful) string type in version 7.

That said, it is nice that I can take a Prolog text from the 1980s or 1990s and find that almost all of the code still works, with minor or no modifications...