Translation:My horses are fast.
Would מהיר be the singular for מהירים?
Sounds like a crass idiom.
Could this also be "My horses hurry" or "... are hurrying"?
No. That would be הַסּוּסִים שֶׁלִּי מְמַהֲרִים
Should "I have fast horses" be acceptable?
That would be:
"יש לי סוסים מהירים."
I tried "My stallions are fast." and it was marked wrong. Why should that not be correct?
A stallion is an unfixed adult male horse - a stud. That's rather specific. But, steed should work.