Perroc forms

How do you get the square and pointed form? Do they hatch? Do I need an item? Is it egg placement?

the form change items perroc needs are a brick piece (sedimentary), magmariser (igneous) and oval stone (metamorphic, which is the default form)!

Try feeding your Pet Rock a Magmarizer or Brick Piece! They will consume the item when they change forms, like how Oricori consumes Nectars

Worth noting that this is explained on Perroc's Dex entry.