Someone crazy about something can be called a fancier. If you join a club to promote the well-being of pheasants, someone might describe you as "a pheasant fancier." That just means you like pheasants a lot.
The word fancier can refer to someone who fancies (or really likes) something — such as dogs or wine or chocolate. It can also be used to describe someone who breeds a particular animal, with the intent of improving the breed. Make sure you don’t confuse this word with the adjective fancy, which has a comparative form that is also spelled “fancier.” That fancier would be used in the sense of “more ornamental” or “more extravagant” — as in "My prom dress is fancier than yours."