Seems like a two part task. You're trying to create riddles and prophecy that works in the context of your game. And you're also trying to write poetry.
As far as writing poetry goes, there are all sorts of resources online and off--try googling "writing poetry" for starters. I'd suggest reading poems written in the style you want to use. Don't just pay attention to the rhyming words, but also to the meter (arrangement of stressed and unstressed syllables within each line) and rhyme scheme (which lines in the poem rhyme with which other lines). If you're looking to employ archaic vocabulary, read some shakespeare, or even chaucer.
You might also take a look at Anglo-Saxon poetry. It emphasises alleteration over rhyme, and makes use of metaphoric devices known as kennings.
Kennings are actually pretty useful for the "cryptic riddle/prophecy" part too. They're figurative phrases used in place of a more literal single word. So you might say "Sail-Road" instead of "sea" or "Spear-Storm" instead of "battle".
The important thing to do when including prophecy in a campaign is to not restrict your player's actions. The two big ways to do this and still keep your prophecies intact are to use open-ended prophecies and a technique called "magician's choice".
Open ended prophecies are prophecies that remain true for a variety of different outcomes. The most famous is probably the prophecy that the Oracle at Delphi delivered to
King Croesus.
The magicians choice is a choice that always results in the same outcome (whichever card you pick from the deck, it will be a jack of spades). Never do so in a way that makes your player's actions meaningless, though.
I told my players at the beginning of my current Dark • Matter game that the world will end in 2012. Eventually they learned that one party member would be responsible for the world's destruction.
But I planned about 5 different ways that the world might end, and multiple reasons that the character in question might choose to end it (I also assigned this fate to the character that is the least likely to be in harm's way during a combat--if he died early, that prophecy wouldn't have worked out very well.)
Finally, sometimes prophecies don't come true. And that's okay.