ผีเสื้อ

From ผี (pǐi, “spirit”) +‎ เสื้อ (sʉ̂ʉa, “tutelar”).

As for the second sense, the insects got their name due to the old belief that they embody ancestral or tutelar spirits.[1]

As for the third sense, the fish got their name probably due to their appearance or colours that look like butterflies.

ผีเสื้อ • (pǐi-sʉ̂ʉa)

1. (classifier ตน) tutelar spirit.
2. (zoology) (classifier ตัว) butterfly: insect of the order Lepidoptera.
3. (zoology) (classifier ตัว) butterflyfish: fish of the family Chaetodontidae, except โนรี (noo-rii), Heniochus spp.