I also found na-adj rather than it's i-form(like 大きな instead of 大きい); ki-adj than it's i-form (美しき instead of 美しい) is used more often in song, poem, or ancient writing.
I have a question, (hope it's not of topic).
I often found 聖なる (can't recall more exs atm) also used in a poetic nuance. Is なる in 聖なる also another kind of adj?
If so, then how about 神聖なる、偉大なる ?
And, could any1 give more examples?