Loads of apps and sites. Go to the Play Store and search for "butterfly", and then consider that that will only find apps with butterfly in the name, and there will be many more which use a butterfly icon but don't have it in the name.
If you can provide a screenshot then there's a remote chance that someone will recognise it, but as there are literally millions of apps in the Play Store the chances of someone who has that particular app seeing your post are not good (unless it's a common/well-known one, but I can't think of one of those that uses a butterfly icon).
So to be honest your best chance is to ask the person on whose phone you saw it.