Great saying! Should be a Brokieism if it's not. Is there such a saying in Germany?
Not sure what you're getting at. This version of hold your horses is mine
, if that's what you mean.
In German, we also have sayings along those lines and connected to horses:
"halt die Hufe still" = hold your hooves still. Slightly different meaning than hold your horses; in German it can mean to wait but also to be quiet/not to fidget.
We also have "Immer langsam mit den jungen Pferden" = take it slow with young horses, which also means not to rush something.