I'm pretty sure "imaginatus" would be fine for "imagined".

There are lots of genera of fern -- indeed, multiple classes -- so clearly (1) you can't just steal an existing genus name and (2) any Latin word broad enough to mean "fern" isn't going to be suitable as a genus name, if that's the intention.

Perhaps "pteridus", which is post-classical but is the origin of the combining form pterido- as in "pteridophytes", the category of plants that includes ferns and various fern-like things. I think its original meaning might be "bracken".

(Note: I am not actually either a classicist or a botanist; treat the above with caution.)