I don’t think you will find anything specifically about designing such creatures (especially such a wide range of them), firstly because it’s an unusual area and secondly because the actual form of such a creature comes entirely from an artist’s imagination. What I suggest you do is study basic drawing techniques and things like human and animal anatomy, which will enable you to build creatures easily and with a basis in realism – even fantastical things need that. Bear in mind that even the best internet tutorials won’t be anywhere near as useful as books or DVDs by good teachers of drawing, so you may want to reconsider that – remember libraries have books too.

I think the best I can offer you is this:
(a in-depth art tutorial geared toward anthro artists, and so it includes information on animal anatomy which should be useful to you).

Good luck.