I have never heard of a deputy crown prince before, but that was the title given to Prince Muqrin of Saudi Arabia on Friday, thus making him second in line to the throne after his older half-brother, Crown Prince Salman.
There is no fixed succession in the Kingdom of Saudi Arabia, but the King appoints a crown prince with the agreement of a council of princes, called the Allegiance Council. Since the death of King Abdul-Aziz (better known as Ibn Saud), the founder of Saudi Arabia, in 1953, the crown has passed among his many sons. Prince Muqrin is the 35th and youngest surviving son of King Abdul-Aziz, which seems to imply that the shift to the next generation - the many grandsons of King Abdul-Aziz, will occur after his future reign.
The current King, Abdullah, who is believed to turn ninety this year, succeeded his elder brother Fahd in 2005. He has already outlived two crown princes: Sultan, who died in 2011, and Nayef, who died in 2012. His third crown prince, Salman, appointed in 2012, is believed to be 78.
Born on 15 September 1945, Deputy Crown Prince Muqrin is considered to be a close ally of King Abdullah, who appointed him director general of the intelligence agency after his accession in 2005. He stepped down from that post in 2012, but was appointed Second Deputy Prime Minister on 1 February 2013 (the King is Prime Minister, the Crown Prince First Deputy Prime Minister).