When does a hunter's pet learn their ability?

[:1]I have a level 10 hunter that I plan to level through the bgs, so I picked up a level 10 spider (Mother Fang, rare in Elwynn Forest). The spider doesn't know the web attack that roots, and I can't find it anywhere in the spell book.
Am I missing something? When does a hunter's pet learn their type specific ability?
No one has an answer for this?
Do I have to wait until level 20? I heard family abilities have nothing to do with the level on the pet. According to beast lore the spider should have Bite, Growl, Web, and Cower, but it only knows the first two, it doesn't know Web or Cower.

Do I have to wait until level 20?

Yes.

I think there might be a specific bat that gets its ability at level 19, but I can't recall what one or even if that was just a rumor.