1) Dual 2) Formed 3) Having or combining two forms 4) Like a mermaid or centaur 
• (a.) Having two forms, bodies, or shapes.
having or appearing in two dissimilar guises. The term is used of characters in classical mythology that appeared to mortals in other than their ...
Bi'form adjective [ Latin biformis ; bis twice + forma shape: confer French biforme .] Having two forms, bodies, or shapes. Croxall. 
[adj] - having or combining two forms
adjective having or combining two forms; `a biform crystal`; `the biform body of a mermaid`
having or combining two forms, as a centaur or mermaid.