I have a pair of rain pants for if I have to be outside for a very long time in the rain, just a simple waterproof nylon pant. Mine are Lands’ End and were bought on clearance, so a nice price, and I usually would wear them over jeans.
Normally, though, I’m not out that long and it’s not that wet if I am. In that case, a parka length jacket is usually fine for keeping my legs warm and dry enough in regular jeans and my knee-high rubber boots.
The other direction would be an even longer coat. You can get them closer to knee length or even mid-calf, and there are riding styles that are divided for riding if you want double duty. I usually buy a Gore-Tex shell and a little large, and then I can layer underneath as needed.