This is the 5836th most frequent Japanese word.
ひょっこり
"ひょっこり" means "to appear or pop up suddenly and unexpectedly."
In this sentence, 『ひょっこり』 is used to describe the abruptness and unexpectedness of someone's appearance.
彼は部屋にひょっこり現れた。
He suddenly appeared in the room.
『ひょっこり』 conveys an image of something subtly and unexpectedly becoming visible.
湖の水面からひょっこり彼女の顔が見えた。
Her face could be seen peeking above the surface of the lake water.
Here, 『ひょっこり』 emphasizes the sudden yet gentle action of the cat's head appearing.