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A lot of them have special heads, or have something to do with heads or eyes
>Exeggcute
Lots of heads
>Hypno
Hypnotises through sight
>Slowking
Shell on its head
>Unown
It's an eye with some stuff attached
>Xatu
Always staring
>Girafarig
Two heads
>Espeon
Jewel in its head, same with Grumpig
>Claydol
Eyes
>Jirachi
Third eye, Medicham implies this too
>Musharna
Dreams coming out of its head
>Solosis
A head in a bubble
>Beeheeyem
Big alien head
>Espurr
The eyes
The designs that don't follow this just tend to have something "off" about them. Psychic is the designated weird type, which is why Psychic Terrain is described as "weird"