Onomatopoeia is the use of words that imitate the sounds they describe. The focus of an onomatopoeia poem is on creating vivid imagery through sound, not necessarily on rhyme.
Here are some examples of onomatopoeia poems that don't rhyme:
Example 1
*Sizzle* of bacon,
*Crunch* of morning toast,
*Glug* of orange juice,
*Tick-tock* of the clock,
*Chirp* of the robin,
*Whoosh* of the wind,
*Rustle* of leaves,
*Pitter-patter* of rain.
Example 2
*Thump* of the bass drum,
*Screech* of the guitar,
*Click* of the cymbal,
*Bang* of the snare,
*Whirr* of the electric guitar,
*Boom* of the amplifier.
While rhyme can add to the rhythm and musicality of a poem, it's not essential to the core concept of onomatopoeia.