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[Slapperfish]

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[Tsuki]

AFAIK, the quest that summons them was removed, so even if it was tameable, there'd be no way of accessing it outside of someone having the quest from the past. But yeah, they could just make a new one...

[Kalliope]

AFAIK, the quest that summons them was removed

Not according to Wowhead. It's just a low chance of the quest turning up as a daily.

[Lisaara]

The daily is still there. I got it back during cata when I was trying to grind some rep there.

[SpiritBinder]

Why is it still untameable? I probabaly because when they flicked the switch on the ooze and hydra it did the same for the ghost croc. It would require someone to go back in a turn on the tameable stitch just for the croc, which... they probably did have time for/not aware of/give a rats ass.

I would be nifty and not at all out of place to have the transparent croc tameable, there are a lot of them these days, it's most likely just been forgotten about. That or it could have been a lot more complicated and we just don’t know. (though I'm guessing the former)

[Wain]

Probably because ghostly pets are still kind of borderline.

We have a few that are ghostly/spirit, but it hasn't always been considered reasonable to tame what amounts to a ghost. Back when that guy was created the idea was still new. Since then things have changed and he doesn't seem so unreasonable.