Is Desghidorah an undeveloped version of Keizer Ghidorah?

No, they are both from a different timeline. Desghidorah was defeated by Mothra, reawakened, then defeated by Mothra Leo. Keizer Ghidorah first arrived in 2004 as predicted by the Xiliens and started off as a monster known as Monster X which later evolved into Keiser Ghidorah. I hope I cleared somethings up for you.

^Truth.

Yeah, what KaijuWorld said.

Also, Desghidorah shoots fire as his primary beam attack. Keizer Ghidorah shoots off gravity beams.

Basically what everyone else said. And Desghidorah is red/grey and Keizer is yellow like King Ghidorah.

Short answer? NO

Same as everyone else said.

ditto

one's black/grey and the other ones has "King" in his name and is yellow.

Flyleaf-Rapt0r I dunno, I've always seen Keizer as a sort of dirty gold/black color.

Ditto. Definitely not the same.