Mercenary
The BBC reported on 10 September 2004 that in Zimbabwe:

[Simon Mann], the British leader of a group of 67 alleged mercenaries accused of plotting a coup in Equatorial Guinea has been sentenced to seven years in jail... The other passengers got 12 months in jail for breaking immigration laws while the two pilots got 16 months...The court also ordered the seizure of Mann's $3m Boeing 727 and $180,000 found on board.

Libya: Muammar Gaddafi in Libya was alleged to have been using mercenary soldiers during the 2011 Libyan civil war, including Tuaregs from various nations in Africa. Many of them had been part of his Islamic Legion created in 1972. Reports say around 800 had been recruited from Niger, Mali, Algeria, Ghana and Burkina Faso. In addition, small numbers of Eastern European mercenaries have also turned up supporting the Gaddafi regime. Most sources have described these troops as professional Serbian veterans of the Yugoslav conflict, including snipers, pilots and helicopter experts. Certain observers, however, speculate that they may be from Poland or Belarus. The latter has denied the claims outright; the former is investigating them. Although the Serbian government has denied that any of their nationals are currently serving as mercenary soldiers in North Africa, five such men have been captured by anti-Gaddafi rebels in Tripoli and several others have also allegedly fought during the Second Battle of Benghazi. Most recently, a number of unidentified white South African mercenaries were hired to smuggle Gaddafi and his sons to exile in Niger. Their attempts were thwarted by NATO air activity shortly before the death of Libya's ousted strongman. Numerous reports have indicated that the team was still protecting Saif al-Islam Gaddafi shortly before his recent apprehension.
Amnesty International has claimed that such allegations against Gaddafi and the Libyan state turned out to either be false or lacking any evidence. Human Rights Watch has indicated that while many foreign migrants were erroneously accused of fighting with Gaddafi, there were also genuine mercenaries from several nations who participated in the conflict.
More recently in 2020 at least several hundred mercenaries from the Russian Wagner Group have been fighting on the side of the warlord, General Khalifa Haftar, whom the government of Russia supports. The Wagner Group mercenaries arrived in Libya in late 2019. The Wagner Group have excelled as snipers, and one result of their arrival was a rapid increase in the number of sniper deaths on the opposing side that holds Tripoli. In response, the government of Turkey has hired 2, 000 Syrian mercenaries to fight for the opposing faction that it is supporting in the Libyan civil war.
Since 2019, Turkey deployed Syrian mercenaries in the Libya (See: Turkish military intervention in the Second Libyan Civil War). In July 2020 Al Arabiyah reported that Turkey sent Syrian, Tunisian, Egyptian and Sudanese mercenaries into Libya.
A November 2020 report by human rights advocacy group Human Rights Watch claimed that approximately hundreds of Sudanese men were hired by an Emirati security firm Black Shield Security Services as security guards for malls and hotels in the UAE, but were subsequently tricked into fighting in the Libyan Civil War. Reportedly 390 men were recruited from Khartoum, out of which 12 spoke to HRW and told that they were made to live alongside Libyan fighters aligned with UAE-backed General Khalifa Haftar. The recruits were hired to safeguard the oil facilities controlled by the Haftar forces.

Middle East: 

Egypt: By 1807, Muhammad Ali the Great, the Albanian tobacco merchant turned de facto independent Ottoman vali (governor) of Egypt had imported about 400 French mercenaries to train his army. After the end of the Napoleonic wars, Muhammad Ali recruited more mercenaries from all over Europe and the United States to train his army, through French and Italian veterans of the Napoleonic wars were much preferred and formed the largest two groups of mercenaries in Egypt. The most famous of Muhammad Ali's mercenaries was the Frenchman Joseph-Anthelme Sève who set up the first staff school in Egypt and served as the chief of staff to Ibrahim Pasha, the son of the vali and his favorite general. By the 1820s, Muhammad Ali's mercenaries had created a mass conscript army trained to fight in the Western style together with schools for training Egyptian officers and factories for manufacturing Western style weapons as the vali did not wish to be dependent upon imported arms.
Muhammad Ali's grandson, Ismail the Magnificent, who ruled as the Khedive of Egypt between 1863 and 79 recruited mercenaries on large scale. After Napoleon III made an unfavorable arbitration ruling in 1869 about the share of royalties from the newly opened Suez canal, which cost Ismail 3, 000, 000 Egyptian pounds per year, Ismail came to distrust his French mercenaries, and began to look elsewhere. A number of Italian mercenaries such as Romolo Gessi, Gaetamo Casati, Andreanni Somani, and Giacomo Messedaglia played prominent roles in the Egyptian campaigns in the Sudan. Ismail also recruited British mercenaries such as Samuel Baker and the Swiss mercenaries such as Werner Munzinger. After 1869, Ismail recruited 48 American mercenaries to command his army. General Charles Pomeroy Stone, formerly of the United States Army, served as the chief of the Egyptian general staff between 1870 and 1883. Ismail's Americans went to Egypt largely because of the high pay he offered, through several were Confederate veterans who were barred from serving in post-1865 United States Army; the fact that the Americans in Egyptian service had fought on opposing sides in the Civil War was a source of recurring tension as the antagonism between North and South continued in Egypt.

Syrian civil war: The Free Syrian Army claimed the Bashar al-Assad regime recruited mercenaries from Iran, Hezbollah militia, and the Iraqi Mahdi Army militia during the Syrian civil war. The Russian government had approved of the deployment in 2016 of the Wagner Group mercenaries to fight for the Syrian government. The Wagner Group is reported to have played an important role in helping to turn the tide of the Syrian civil war in favor of the government, which in 2015 appeared to be close to collapse. On 7 February 2018, the Wagner Group mercenaries were reported to have attacked an American base in Syria together with a pro-Assad militia in what is known as the Battle of Khasham.
Turkey used Syrian mercenaries against the Kurds in Syria.

Yemen civil war: Multiple mercenary groups, called Popular Committees, which consists of Yemeni tribes loyal to different factions, were formed by both the Hadi government as well as the Houthi Supreme Political Council in the Yemeni Civil War.

Saudi Arabian-led intervention in Yemen: During Operation Decisive Storm, multiple sources reported that Latin American military contractors from Academi headed by Erik Prince were hired by UAE Armed Forces to assist in the fight against Houthis.

Notable mercenaries: 

Cyber mercenary: A cyber mercenary is a non-state actor that carries out cyber attacks for Nation states for hire. State actors can use the cyber mercenaries as a front to try and distance themselves from the attack with plausible deniability.

See also: Filibuster (military)
International Stability Operations Association
Mercenaries in popular culture
Mercenary Soldiers' Revolt in Brazil
Mercenary War (c. 240 BC) – also called the Libyan War and the Truceless War
Montreux Document
Private defense agency
Private intelligence agency
Privateer
Rōnin
Security detail
Military volunteer
List of foreign volunteers
Violent non-state actor

References: 

Sources: Arrian (1976). de Sélincourt, Aubrey (ed.). Anabasis Alexandri (The Campaigns of Alexander). Penguin Books. ISBN 978-0-14-044253-3.
Bernales-Ballesteros, Enrique; UNHCHR: Special Rapporteur of the Commission on Human Rights on use of mercenaries
Bodin J; Les Suisses au Service de la France; Editions Albion Michael, 1988. ISBN 2226033343
Bryant, G. L. (January 2000). "Indigenous Mercenaries in the Service of European Imperialists: The Case of the Sepoys in the Early British Indian Army, 1750–1800". War in History. 7 (1).
Chartrand, Rene; Louis XV's Army – Foreign Infantry; Osprey 1997. ISBN 185532623X
Chartrand, Rene; Spanish Army of the Napoleonic Wars 1793–1808; Osprey 1998. ISBN 1855327635
Goldsworthy, Adrian (2006). The Fall of Carthage: The Punic Wars 265–146 BC. London: Phoenix. ISBN 978-0-304-36642-2.
Milliard, Todd S.; Overcoming post-colonial myopia: A call to recognize and regulate private military companies Archived 1 December 2012 at the Wayback Machine(PDF), in Military Law Review Vol 173, June 2003. At the time of publication Major Milliard was a Judge Advocate in the United States Army Judge Advocate General's Corps
Anthony Mockler, Storia dei mercenari: Da Senofonte all'Iraq. Odoya, 2012. ISBN 978-8862881531.
Schuster, Carl Otis (2016). "Memnon and Mentor of Rhodes (ca.