What other things can mimic an epididymal cyst on an ultrasound or is an epididymal cyst on an ultrasound definitely always benign?

Benign: Epidydimal cysts are very common and always benign. No worries.

No such thing as: "definitely always", but in this case you are close. Malignant tumors of the paratesticular structures are rare but not impossible. These would likely grow more rapidly than the common benign cyst.

Benign things: The differential diagnosis of benign epididymal masses includes epididymitis, spermatoceles, varicosities, adenomatoid tumors, and infections. Malignancy of the epididymis is very uncommon.

Almost always benign: A spermatocele can commonly mimic an epididymal cyst. Both, though, are benign and are not typically followed up.