Pathfinder 1E How to obtain a Dire Paladin Mount or a Dire Animal Companion?

I have a player using a Dwarf Paladin PC that wants to eventually own a Dire Boar Paladin Mount. Out of curiosity, I'm also wondering if this can be accomplished with Animal Companions? Is there an official way of handling this in Pathfinder?

I can also use 3.5 sources if I need it. If there is no official way, I can settle on house rules if anyone has created a feat that does this.

He mainly just wants a large sized boar so he can ride it. But the boar animal companion only becomes a medium size animal at Druid level 4. So I'm wondering if there is a way to advance it similar to a dire boar at maybe level 7? I see that a lot of other animals become large size at level 7.

I don't see any rules for it. So, It's take the stats one of those animals that turns large and reskin it to be a dire boar.