Suspicious Grinning Eye

At rest it is a small blue eye that hovers near you. Push past roughly the speed of a decent pair of wings or a fast mount and the pupil slides up out of the way and a mouth appears underneath, copying the boss version's second form. On a late game character with real movement gear it spends most of its time grinning, which is where the name comes from.

Standing still, it parks itself ahead of you. Equip the Suspicious Looking Tentacle and the eye gets shoved to your other side while the light pet takes the front. If you want the pair to look like matching siblings, teal dye on the pet gets them close enough to confuse.

The Eye of Cthulhu only hands this over on Master, so it is a reason to open at least one Master world even if you have no interest in playing the whole run there.