The genus Pachypodium belongs to the family of the Apocynaceae. There are two seperate areas where they are native: Madagascar and Angola/Namibia. There is some confusion about the number of species. Fred Triep counts 13 species of which 4 from Namibia/Angola and 9 from Madagascar. Wikipedia totals 25 species of which 20 originate from Madagascar. The truth might be somewhere in between for we have the impression that wikipedia also mentions some hybrids.
The most commonly known Pachypodium in South Africa is the P. namaquanum (halfmens) from Namibia. This is a very slow growing specie (in its habitat max. 2 cm per annum) and many nurseries unfortunately sell the fast growing P. lamerei from Madagascar as the ‘halfmens’ (= ‘half human’ because of its shape). The P. namaquanum is a protected specie in South Africa.
Another interesting specie is the P. succulentum (also from Angola/Namibia). Although not on the list of endangered species it’s less common in its native area than the P. namaquanum.