RFID Reader for Mobile Devices
Before you start any work please familiarize yourself with this:
http://en.wikipedia.org/wiki/ISO_11784_%26_11785
and this: http://en.wikipedia.org/wiki/ISO_14223
RFIDs for animals are completely different protocol than NFC. It is more like UNIQUE tags (125kHz) . Anyway above ISO standards will be a good starting point for you.
Android have native support for NFC, you don't need any bluetooth adapter or anything else:
http://developer.android.com/guide/topics/connectivity/nfc/index.html
Not all devices have it, but the major ones do have (e.g. galaxy nexus, nexus 4, galaxy S3, HTC One) and it works nice.