Is my dog invincible or is it a glitch?

[Lightningfalcon]

Doing some quest for a shout in folgruther whatever it's called and on the last boss with lots of adds, i ran into the boss with my dog and i went straight for the boss first and when i nearly killed him i turned around and all the adds were dead with my dog was jumping around.
before i bought the dog from markath [viligance] the internet said the dogs dies easily, i'm a bit confused.

 

[Dagmar]

Unlike the other 2 mortal dog followers, Meeko and the Stray Dog, who level cap at 25, Vigilance's level cap is 50. The later you first visit the Markarth Stables, the higher level Vigilance will be and he will be much more durable than other dogs if you don't go there until you are level 40 or above (he'll have 400+ health).

Even the other dogs are pretty sturdy in leveled encounters where you are level 25 or below. It's at higher levels that they tend to die easily.