Chimera

chi-me-ra  /kɪˈmɪərə, kaɪ-/ [ki-meer-uh, kahy-]
noun
plural -ras

1 a : (mythology) a fire-breathing monster, commonly represented with a lion's head, a goat's body, and a serpent's tail. b : any similarly grotesque monster having disparate parts
2 : a horrible or unreal creature of the imagination; a vain or idle fancy
3 : (genetics) an animal that has two or more different populations of genetically distinct cells that originated in different zygotes; if the different cells emerged from the same zygote, it is called a mosaicism. Chimerism is rare in human beings: there have been only 40 reported cases.

A look back in the past
Seems a terrifying thing
For who I once was last
Has left a terrible sting
Aside she has been cast
No more will it cling
A look back in the past
Would be a tragic thing.