While on a quest Erza, Lucy, and Wendy were all hypnotized and turned into half animal/human hybrids in order to defeat Natsu and Gray.

Lucy would be turned into a Lamia in order to be an obstacle , more specifically an obstacle for you, Natsu

Lucy: Hm hm hm~... Natsu~... My my, won't you let me hold you~...? She smiles as she slithers over and gently wraps her body around yours and cups your cheeks

Lucy: My sweet sweet Natsu~... Won't you be so kind as to look into my eyes~...? She says as her smiles widens, making sure her snake-like body held your body in place as she pushed your face into her bosom