How do I find Shadowmere again if I lose her?

This is a tricky one as it's possible to actually bug out Shadowmere and have her disappear from the game completely.

A couple of different steps to try:

• Fast Travel to a known location, like a stables. She may appear with you.
• Travel to Shadowmere's "home" at Fort Faragut.
• Enter a town and leave again, have a look in the stables. She may be stabled there.

As I said, it is possible to completely lose Shadowmere. If your on PC there is a console command that will spawn a new instance of her next to you, but I don't know it off the top of my head.

Hope it works for you!

Fast-Travel back to Fort Farragut, and walk right up to the main doors to go in and Shadowmere will be standing there waiting for you.
Shadowmere should always be waiting for you there if you get separated.