translation "golden fleece" [classical Latin]

To begin with, the Latin word for "golden" is aureus and the Latin word
for "fleece" is vellus.

1. Aurum velleris
"Gold of fleece"

2. Phrixзa vellera
I'll assume you meant Phrixea
"Fleece of Phrixus" or "Phrixean fleece"
Here the plural form (vellera) is used.

3. Aeolium vellus
"Fleece of Aeolus"

4. Aurei velleris
"Of the golden fleece"

5. Aureus velleris
Looks to me like this is also trying to say "of the golden fleece", but the
case and gender of the adjective aureus does not agree with the noun
velleris.

Personally I'd go with the nominative, vellus arureum.

thank you very much for your answer, but can you clarify vellus arureum or vellus aureum?

Good catch, viktor80!
vellus aureum

I'm just paste your version in the search and found two source;

[Historia Argonautarum: Seu Ad Vellus Aureum Expeditio Et de Navigationis Origine]

and

[Antonio Kaschutnig. Vellus Aureum Burgundo-Austriacum sive Augusti & Celeberrimi Ordinis Torqatorum Aurei Velleris Equitum Relatio Historica]

that's why I suspected that is a typo

thank you once again

You're welcome, viktor80.
Thanks for the references.