When a yukkuri's stupidity, overconfidence, superiority complex, and/or limited experience causes them to hold a skewed view of the world that's heavily biased towards themselves. Shitheads (and by that extension deibus and wasa reimus) are naturally delusional creatures though not all delusional yukkuri are shitheads.
A lot of koyukkuris are born delusional but unless they're truly shitheaded these delusions can usually be corrected with good parenting. Conversely a relatively humble or level-headed yukkuri can turn delusional if they're too spoiled which is a particularly common mistake among inexperienced pet owners.
These skewed views vary but the some of the most common examples include:
believing that they're someone special, the chosen one, a hero/king (marisa) or a princess (reimu)
believing that they're the strongest creature in the world, can easily defeat anyone else, and anyone who manages to fight back must be cheating somehow
believing that every other creature (especially humans) exist to serve them as their slaves, making them easy is their responsibility
believing that they cannot make any mistakes at all and if something in their plans went wrong it's someone else's fault
believing that their (or their children's) cuteness means that they'll be forgiven from any wrongdoings that they do
believing that everything that a human have (house, food, clothes, etc) are found/given instead of being earned/bought and thus they must have equal rights to obtain those items too, usually claiming "easiness are meant to be shared!"
believing that saying "don't notice [insert yukkuri type here]" while "sneaking" in broad light and in the open makes them invisible.
believing puffing up is actually scary or even hurtful against humans and animals.
