Puck is either born or bitten and can become an animal like a wolf or a cat. He finds out that is mate is Kurt, only he keeps freaking Kurt out by sniffing him and growling.
I remember that eventually others like Puck come for Kurt because Kurt keeps on denying Puck. Kurt gets scared and does want Puck so he runs to him.
Sue is also like Puck and she helps them get away from those who were after Kurt.
I have read the fic 'Bitten' .....that is not the one I'm looking for.