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It can be top of sheep's head, isn't it?
Did you throw an invisibility potion at the sheep?
It can be invisibility potion. When you trow invisibility potion on spider you will see only its eyes and when you trow potion on sheep you will see only wool (when you shear the sheep it disappear)
Quique Arokip is correct. With sheep, the wool is its "armor," although it has no effect on the damage taken. Since armor is still seen when you drink an invisibility potion, the wool is still seen when the sheep has an invisibility effect on it.
and with the eyes of the spider, they are still visible. This is the same with an invisible enderman

Intentional.
Shear the sheep and they should completely disappear if that was caused by the potion of invisibility.