Well, when the spirit of the Dragon King was freed from that egg, the first body it encountered was Reptile's, wich it pocessed and molded at his will in order to resemble his long lost physical shape.

It's a bit of a story:

It began when Nitara tricked Reptile to help her disable Cyrax's arm console so he could help her acces the chamber of the Last Dragon Egg. Which that chamber also held this orb that would seperate Outworld from the vampire relam.

When she destroyed the orb, Nitara just left, but Reptile, who somewhat felt betrayed by Nitara, followed her and Cyrax and arrived shortly after Nitara managed to transport Cyax back to Earhtrealm and herself to her native realm.

Though at that moment, the egg hatched and sent its energy into Reptile, which kinda fullfilled the prophecy of Onaga's return. So Onaga basically used Reptile's body like a puppet therefore Reptile was transformed only one time into something else in a Mortal Kombat game. Hope that clarifed some things up for you.

i never knew this before, i know the dragon king is the main antagonist in deception, but what game did they reference that hes using reptiles body?

MKDA Reptile. Read his bios and endings and stuff.