This is a Babylonian form; the examples in JPA are almost certainly mis-copied forms of כעין, as shown by TgPsJ at Dt. 33:28, while the Onkelos passage is of a dubious textual background.
In JBA, the only original usage is probably in the very frequent expression מעין שופריה, which is probably best rendered by taking the initial mem literally to mean "some of", so: "something approaching its beauty".