This diagram shows the development of the Rurikid's coat of arms. Some suggest the Tryzub depicts a descending bird of prey. Looking at all of these symbols together does give the impression of this.
Any symbol that predates Christianity probably is pagan. Whether it's Slavic or Nordic gets messy, though. It's suggested that the trident has Christian symbolism in reference to the trinity. This would make sense to me because Vladimir of Kiev christianized the Rus. It was him who added the middle part, making it a trident.
The adoption of the symbol in 1917 is certainly a reference to this Rurikid dynasty of Kiev. The origins of the Ukranian people actually began with the breakup of that principality, though. This is when regional differences in language began and Kiev struggled with Russian princes to the north and east (1).
(1) Vernadsky, George. Kievan Russia. Yale University Press, 1948 p. 215