It’s extremely rare, but that isn’t the main complaint, the complaint is that it conflicts with Hunters who want to tame it. I feel bad even camping for it because i know some hunter probably wants to tame it if it spawns. My point is, the rare is EXTREMELY rare, never seen it once and there doesn’t appear to be an actual way to spawn it, I’ve seen countless Soundless but no Deepglider. I have needed it for the achievement for weeks now.
Knowing all this makes it harder to camp, because I know hunters will want to tame it as well. 
That was my last one I needed. Very grateful to the guy who announced in chat Deepglider spawned. I killed it on an alt but man that meta achv only being one character only is lame.
According to the wowhead comments on Deepglider.
It sounds like there isn’t much need for hunters to camp it, because there is another skin option that looks like it, and the reason it’s so rare is that no one typically kills the bloated skyrays, and that if you farm them, you may actually get it to spawn in a reasonable time.
This is not confirmed whatsoever. If you read the rest of the comments you’ll see a lot of people disputing these types of comments. I’ve spent the majority of the last week killing bloated sky rays, and hanging around specifically when the daily quests have other people killing rays for me, too. I’ve still yet to see Deepglider spawn.
If you read the comments on wowhead you should have seen people disputing the bloated sky ray farm tactic.