Salam
Why is it that the arrival of the prophet can be found in holy scriptures other than that of the eh'le kitaab? e.g Bible.. Dr Zakir naik brought it up in a lecture about the similarities between hinduism and islam.
And how did this come to be in there, where did it come from
The Vedas and Bhagwatgita do predict Prophet Muhammad. The prophecies related to the final RISHI have been fulfilled by the prophet. Where did they come from? that can be answered by Quran
Chap13 verse 38
وَلَقَدْ أَرْسَلْنَا رُسُلاً مِّن قَبْلِكَ وَجَعَلْنَا لَهُمْ أَزْوَاجًا وَذُرِّيَّةً وَمَا كَانَ لِرَسُولٍ أَن يَأْتِيَ بِآيَةٍ إِلاَّ بِإِذْنِ اللّهِ لِكُلِّ أَجَلٍ كِتَابٌ ﴿٣٨﴾
38:We did send apostles before thee, and appointed for them wives and children: and it was never the part of an apostle to bring a sign except as Allah permitted (or commanded).
For each period is a Book (revealed).
It is possible that Vedas and Bhagwatgita may have a divine origin since Allah gave a book for every period. But we cannot be sure since Quran does not confirm it. And we cannot say that they are completely the word of God, we can only say that they may have partially been the word of God. But even if we assume that they are from God, they tell us to follow the final Rishi i.e Prophet Muhammad, hence we should follow him if we want to obey the Vedas/Bhagwatgita.
Hope I helped