Phenomenon is Greek twice marked: it has the ph that transliterates phi, and it keeps its Greek ending, which is why the plural is phenomena and not phenomenons. The Greek is phainomenon, "that which appears," the neuter participle of a verb meaning "to appear," from a root meaning "to shine" — the same root, at a distance, behind fantasy and phantom.
English took it in the 1570s through Late Latin, meaning a fact directly observed. The sense of an extraordinary occurrence is later, from 1771, and the philosophical sense — an appearance, the immediate object of experience, as opposed to the thing in itself — from 1788, arriving with Kant. Like criterion and criteria, the Greek plural is under pressure in English, and phenomena is routinely used as a singular by speakers with no reason to know better.
phenomenon (n.) 1570s, "a fact directly observed, a thing that appears or is perceived, an occurrence," especially a regular kind of fact observed on certain kinds of occasions, from Late Latin phænomenon , from Greek phainomenon "that which appears or is seen," noun use of neuter present participle of phainesthai "to appear," passive of phainein "bring to light, cause to appear, show" (from PIE root *bha- (1) "to shine"). Meaning "extraordinary occurrence" is recorded by 1771. In philosophy, "an appearance or immediate object of experience" (1788). The plural is phenomena . also from 1570s