What am I?

*I personally don't know anything about it

Add a small insect and you have a 1994 Indian action film*

@aguy found these two parts,

fish -> h -> anth

Take something you have when you don’t have something else, remove its last letter and add it.

lack -> lacanth

Rearrange a CEO and stick him on.

ceo -> coe -> coelacanth

Apparently they thought I was... what did the geriatric egg do?

THE EGG STINKED / EXTINCT. Coelacanths were thought have gone extinct with the dinosaurs, but then rediscovered alive.

• Well done! This is the correct answer. – hat Dec 15 '18 at 18:25

Not done yet...

FISH is both ectothermic and edible.

Remove three letters.

H

Add a small insect and you have a 1994 Indian action movie

ANT $$+$$ H $$=$$ ANTH

I didn't get any further than this...

Partial:

Probably fish. The fish is ectothermic and edible.

Remove three letters.

Removing the first 3 letters gives us h

Ant. And we get Anth which is the name of an Indian movie from 1994

Take something you have when you don’t have something else, remove its last letter and add it.

What you have when don't have something else is you miss something. Apparently this is incorrect (due to the comment by the author).

Rearrange a CEO and stick him on.

So, from CEO we can get coe, oec, oce, eco, eoc.

• I'll leave you to work it further. We'll see if you are on the right track or not. Maybe I'll add hints later if it becomes necessary. – hat Dec 15 '18 at 12:35
• See edit....... – hat Dec 15 '18 at 16:33
• rot13: Abgr gung lbh pna'g unir n zvff va gung frafr. – hat Dec 15 '18 at 16:51