Perfect rhymes:
* Arcus (a type of rainbow)
* Focus
* Octopus
Near rhymes (slant rhymes):
* Markus (a common spelling variation)
* Carcus (an old word for a carcass)
* Arcas (a figure in Greek mythology)
Eye rhymes (words that look like they rhyme but don't):
* Marquis (a noble rank)
Let me know if you'd like more rhymes, or if you're looking for something specific!