Beast Rider Cavaliers and huge mounts. How to get them?

[Sereinái]

The beast rider Exotic mount feature states the following:
"In addition, a 7th-level or higher Medium beast rider can select any creature whose natural size is Large or Huge, provided that creature is normally available as a Medium-sized animal companion at 7th level (like a bear). " (source: PRG)
Yet I can't find any druid companions that are huge let alone ones that begin as medium and no rule for how to bump them up to huge size like you can with medium to large. What am I missing here?

I'm a bit saddened that I can't use the majority of mounts unlocked at level 4 as medium sized character until level 7 due to medium to large growth happening later than small to medium but as how are you supposed to deal with the part about huge mounts?
Should one apply the Large conversion onto a large animal and if so can I do that on a creature unlocked at level 4 (7) that has a superior level 7 advancement naturally?

I don't think the part about it being able to be huge is a mistake as it earlier is stated that medium characters need a mount of size large or huge in order to ride

[JiCi]

Let me get this straight: you're looking for a Huge animal that would normally start as a Medium companion and then get Large at 4th level so you could increase it to Huge at 7th level... right ?

[One]

Something like a mammoth perhaps?

[Mojorat]

There is no animal companion that has Huge as its natural state short of house rules.

[Sereinái]

Ah. I hadn't thought about companions outside the core and APG, that might introduce some huge ones.

What does anyone know some other interesting huge ones?
I'm probably going to take something like an elk or bear for the flavor of them but was interested in what sorts of towering monsters one could end up riding into battle.

[mdt]

I always read that ability as allowing you to take the same type of animal, as long as it's already available as a medium sized animal companion.

So, bears are available as a medium sized animal companion, therefore you can pick a large or huge bear at 7th level from the bestiary. Not from the druid AC list. Or a smilodon (large cat), or a dire wolf, etc.

[Stikye]

I always read it as taking the bear, adding the extra bonus from it and turning it from medium to large in order to be able to ride it. Its still an animal companion.

So the bear example, its a small animal companion, at 4th level its a medium with all those boosts, and 7th level if you want, you can turn it into a large bear animal companion since there are large bears in the wild. Since turning it into a large bear would let you ride it.