[RPG] What does body type mean in the disguise self spell description

First Monstrous humanoid is not a subtype of Humanoid. Check the back of your bestiaries for creatures by type.

The emphasis on not changing creature type is based on the extent of the illusions power.

This is only a level 1 illusion spell the extent of the changes are quite limited to only minor things within reason. Obviously when compared to more powerful illusions the area and effects are greater. Within reason try to consider each level to be the cutoff of power for each. Ex. invisibility is an illusion + glamer, but it is higher level than disguise self so this would indicate that disguise self is not entirely capable of doing something as complete as full body invisibility.

A typical type of creature from another is so different that it would be too obviously different. Like a Human in disguise as a lizard man. The tail and bone structure are so different that you would look like a person glitching through the character model that is being overlapped on top of you. This spell is only an illusion with the Glamer type.

Glamer: A glamer spell changes a subject’s sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.

In general if you go outside of the bounds of the illusion or are disrupting it in some way it either fails or can be seen through.

Alright, let's start saying that your assumption to apply real world physics to explain how unexplained spell effects work isn't going to work very well, specially in D&D/Pathfinder.

Now, Pathfinder has an two unwritten rules that say: "unless something says you can do it, you can't." and "specific rules override the ruling of the generic rule; and when lacking a rule, use the general ruling". As I said, those are unwritten (yet commonly accepted) but will help you a lot when reading things as written, and I need to mention them to further explain my case bellow.

Now, let's tackle those illusion spells that cause so much confusion at every table. The spell description usually says what can be seen, anything else you should check the rules for Illusion in the Magic chapter, and if it can't apply, the general rules for spells (you would be surprised how many people actually incorrectly use certain spells by ignoring that).

Illusions says:

Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened.

Disguise Self is a glamer effect:

Glamer: A glamer spell changes a subject's sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.

That said, we can see that even being an illusion, Disguise Self would actually make you feel like the illusion is real if you fail your saving throw. But in the case of Disguise Self, the part about tactile (touch) and audible (sound) perception isn't changed like normal glamers.

As for disbelieving it, Saving Throws and Illusions (Disbelief):

Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion. A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.

Notice how glamer isn't listed here? So you don't get a translucent outline at all, it simply disappears to your character.

So, even if you can touch and feel the target if you reach out, the illusion works just as written, it doesn't clip into buildings, it doesn't fall into the ground, it doesn't levitate because the item is worn by a smaller creature pretending to be a large one. How would an ogre feel when disguised as a halfling? If you tried to touch him you would feel something on the way (the ogre's body) and would most likely get a saving throw to notice the illusion and see the real ogre. But on a failed save, the GM could say you feel a force field protecting him (or something along those lines).

As for a dog wearing a hat of (animal) disguise, it would look like a normal person walking around, even dodging obstacles so the illusion can remain convincing. You would only get a saving throw if you tried to touch that person, or if the dog barked (a human barking? what is going on here...).

Why? Because the spell description does not say so. It isn't intended to do so; if that was the case they would have spelled it out to work that way. The spell doesn't work like a hologram that has a fixed center of emanation and must be correctly placed or otherwise it would "look weird", because the spell is altering the senses of those who see it. For contrast, I believe the technology guide does have an illusion spell that works exactly like a hologram, by the way.

Happy gaming. :)