According to Ptolemy's writings it was a 4 months journey south of the Fezzan and contained large animals, such as rhinoceroses and elephants, as well as many tall mountains. Ptolemy's account is based on that written by Marinus of Tyre between 107 AD and 115 AD. Between the years 83 AD and 92 AD, the king of the Garamantes claimed that the Agisymba population were his subjects. Modern historians suggest that Agisymba was probably somewhere to the north of Lake Chad as the Chad area is populated with many mountains. One theory is that Agisymba was an antecedent of the Kanem Empire located on the northern shore of Lake Chad. 
In AD 90 a traveler, probably a trader called Julius Maternus, profiting from the improved relations between the Romans and the Garamantes at this time -no doubts as a result of Flaccus's success- made his way through the land of the Garamantes to the land of Agisymba, where there were rhinoceroses. JD. Fage (Cambridge history of Africa) 
Indeed Ptolomeus wrote that around 90 AD Julius Maternus (or Matiernus) did a mainly commercial expedition. From the Sirte gulf he reached the Oasis of Cufra and the Oasis of Archei, then arrived -after 4 months travelling with the king of the Garamantes- to the river Bahr Salamat and Bahr Aouk, near the actual Central African Republic in a region then called Agisymba. He went back to Rome with a rhinoceros with two horns, that was shown in the Colosseum.
- Agisymba on BrillOnLine Reference Works.
- "The Mune as the Ark of the Covenant between Duguwa and Sefuwa (in ancient Kanem)" Borno Museum Society Newsletter 66-67 (2006), 15-25. (The article has a map (page 6) of the ancient Central Sahara and proposes to identify Agisymba of 100 CE with the early Kanem state).
- Fage."Trans-Saharan contacts and west Africa". The Cambridge History of Africa.Volume 2 (p.286)
- Agisymba and Maternus
- Desanges, Jehan, Recherches sur l'activité des méditerranéens aux confins de l'Afrique, Rome 1978 (here: p. 197-213).
- Huss, Werner: Agisymba, New Pauly, vol I, Stuttgart 1996 (Sp 260).
- Lange, Dierk, Ancient Kingdoms of West Africa, Dettelbach 2004 (here: pp 280-4).
- West Africa and the Classical World - neglected contexts " in H. Bleyet al.(Ed.),in Africa Context: references in world history and the present, 19 international meeting of the VAD, Hannover 2004, p. 20