Aquiring a Mindless type

[Genth]

In my game, I wish for there to be a template which is acquired, which gives the creature the 'Mindless' type. They become an autonomous, violent, ooze-like humanoid (assuming they were humanoid to begin with). A couple of questions on this:

Is there actually a mindless 'type' or descriptor, or is it just vocabulary to describe vermin, undead, constructs and so on?
Does it have a defined in-game meaning, for example, does being described as mindless automatically give immunity to mind-effecting spells?
Would a creature who is given this 'type' lose all their feats and skills? If they had a class, would they lose BAB, HD, saves etc?

[Milo v3]

Mindless is not a creature type, it is an ability oozes and vermin possess.

The effects of being mindless are: "No Intelligence score, and immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects)." and "Mindless creatures do not gain feats or skills, although they may have bonus feats or racial skill bonuses."

[meschlum]

There are a couple Pathfinder templates which do this. They are typically terrible, but they exist. Mostly, the process takes place by turning the victim into an undead creature (Skeleton or Zombie are traditional, other options are present too).

That said, what you're looking for sounds a lot like the ectoplasmic creature template, which is a horrible template as far as assigning CR goes. Only use the CR for bruiser type monsters, as the loss of all skills and spellcasting (and everything that requires more planning than "charge!") isn't really balanced by slightly higher attributes. It's also undead, for what it's worth.

All the templates that involve a loss of all mental faculties also remove feats, skills, spellcasting, etc. You could argue that applying such a template to a stronger body might lead to a stronger opponent (skeletons and zombies explicitly lose this most of the time), so as a rule any levels are lost in the process - though one could argue otherwise in some cases.