Thoughts on comments from the tweet had this:

@XCOM Dog, uses bite with a 99% chance to hit, Still misses, Bites the air behind the alien.

“Scientists have calculated that the chances of something so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.”
-Terry Pratchett

@XCOM Damn yous! As soon as I saw this, I was so excited because I thought it was a thing.

3627 Workshop items, yet XCOM 2 still doesn't have a K9 class....

@XCOM ok, but the dog never dies...doesn't matter what happens...it never dies...ok?

...Probably because of this. Man, I stall-out in XCOM when I worry about the humans dying.

@XCOM it even comes with a song, XCOM - Irondog Possible

BeagleRush's Twitch stream was the source of this (a viewer drew this in response to him commenting about how XCOM ought to have attack dogs).