Drovers coat. Straps to hold it down on your legs, cut in back so that it won't get caught in the saddle. Roomy enough to add a sweatshirt under it. Fleece lining that can be removed. Waterproof.

https://smile.amazon.com/Walker-Hawkes-Stockman-Hooded-XX-Large/dp/B00WZJ9LFY/ref=sr_1_4?crid=27B0E7TCHSBZE&keywords=drovers+coat&qid=1643338836&sprefix=drovers+coat%2Caps%2C148&sr=8-4