What is a Bullmastiff x Border Collie called?

There isn't a specific name for a Bullmastiff x Border Collie mix. It's simply called a Bullmastiff Border Collie mix or a Bordermastiff. You can also use more creative names like:

* Bulldor Collie

* Bordermastiff

* Collie-mastiff

* Bull-Collie

Ultimately, the name you choose is up to you!