See generally Neil Hopkinson in OCD(4) s.v., distinguishing between two men: M. the priest (fl. 280 BCE), author of a (lost) history of Egypt (Aegyptiaca) in 3 books (FGrH 609); a homonym of the second or third century CE to whom are attributed 6 books of dactylic hexameters on astrology, entitled Forecasts (Apotelesmatica -- see under mu 143). See also Waddell pp. x ff. For other works attributed to Manetho, see most conveniently idem pp. xiv ff., xxvi ff.
The name Manetho itself is surprisingly rare. It is firmly associated only with the historian. The astrological writings are either partly by this author or by another using his name. The physician and magician may well be identical with each other, and perhaps pseudonymous.
The etymology of 'Manetho' is controversial. For discussion and bibliography, see Spiegelberg pp.145-150; Waddell p.ix; and Thiessen pp.93-96. In Greek, two legitimate forms are known: (a) *Ma/neqws, -qw, which appears as a dative in a letter of 241 BCE, and may well be a reference to the historian (P. Hib. 1.72.6-7: correctly transcribed by Grenfell and Hunt; wrongly reported by Jacoby, at FGrH III C 609 n., whence Fraser p.728 n.97; the papyrus itself survives in the University Library, Cambridge). (b) *Ma/neqwn, -qwnis, in Corpus Inscriptionum Latinarum 8.1007 (nominative). These names, when they appear in the manuscripts of ancient authors, are variously accented. Probably, for the historian, paroxytone was original. It may also be noted that three illegitimate forms of the name survive: (c) *Maneqw= (indeclinable), in the Book of Sothis, a work once rightly assigned to the close of the third or beginning of the fourth century CE by J.A. Letronne (see Naiditch pp.104 sq. n.2). (d) *Maneqw=q (indeclinable) twice in Africanus (apud Syncell. chron.), a trivialising blunder, though some regard it as etymologically significant. (e) *Ma/neqos, presumably due to a scribe's jibbing at Ma/neqws.
[1] Barrington Atlas map 74 grid F3.
[2] A type of incense (kappa 2797; see also Waddell pp.202 sq. n.2.)