From English:
* Watch - obvious but effective
* Alert - another straightforward choice
* Guard - emphasizes the protective nature
From other languages:
* Aegis (Greek) - meaning "shield", representing protection
* Odin (Norse) - god of wisdom and war, associated with vigilance
* Sentinel (Latin) - a guard or watchman
* Vigilant (Latin) - directly translates to "vigilant" but might be a bit long
Other ideas:
* Jasper - a gemstone associated with protection and strength
* Scout - implies exploration and awareness
* Shadow - suggests stealth and keen observation
* Hawk - a bird known for its sharp eyesight
Ultimately, the best name depends on your dog's personality and your own preferences!