In Pixiv, you can leave a comment on any fan-art. There is also an option to leave a sticker for comment. They include an elephant, fox (?), girls, and skull.
Here is a fan-art example from one of those stickers. I've tried to google the tag but couldn't find anything.
I'm asking about all the stickers - elephant, fox (?), girls, and skull - , but more specifically, the one for these girls above. Do these stickers come from an anime? If so, which one? If they are original characters, do they have any names?