Uhm.... the Genitive case IS possesive, in German you probaly called it the second (name) case.... In Latin we call it the Genitive (Genitivus). Since all Indo-Proto-European language's once had 8 name cases (Nominativus, Vocativus, Genitivus, Dativus, Accusativus, Ablativus, Locativus and Instrumentalis), we have one universal term for it, and that's the Latin one (Or word's taken from the Latin name Nominativus = Nominative in English)....
Oh and I looked it up in my book about Anglo-Saxon, it's -es
So my Anglo-Saxon (Blah.. it's not Rohirric, it's Anglo-Saxon, or Old English) would be Tíwes
The Nouns in Anglo-Saxon, male the a-stem :
Singular
Nom. - Hund
Gen. - Hund
es
Dat. - Hund
e
Acc. - Hund
plural
Nom. - Hund
as
Gen. - Hund
a
Dat. - Hund
um
Acc. - Hund
as