In the text of Negative Energy Flood, it says that a target killed by the spell becomes a zombie at the start of your next turn. The spell does not limit this to humanoids, like Finger of Death. Would this mean that a DM might have to come up with a zombie variant for creatures on the fly? For example, let’s say a spellcaster killed a Androsphinx with Negative Energy Flood. Would the DM have to quickly come up with a zombie variant of its stat block, since it wouldn’t make much sense for the Androsphinx to become a humanoid zombie?

In the event your DM needs some Zombie stat blocks the DMG has a template you can add to monsters to make them zombies, it breaks down as:

Ability Scores modified as: +1 Str, +2 Con, -6 lnt, -4 Wis, -4 Cha

Special Abilities: Undead Fortitude (as normal Zombie); immune to poison damage/poisoned condition; darkvision 60ft.; can't speak but understands the languages it knew in life (so loses pretty much all spellcasting).

 Edit: although you could infer from the spell description that a normal humanoid zombie rises from the corpse of the creature you kill with the spell, so it's upto the DM on how they want to run it.

A DM can simply use the zombie statblock and not have to modify nearly  anything, perhaps size. Or it can modify more like attacks, damage type, traits, abilities etc 

There's also some zombie monster published IIRC

Zombie is not a creature type in D&D as far as I know, although it might be used as a descriptor. So I interpret the spell as creating a creature represented by the zombie stat block.

Rather than altering size, I might make it only a particular part of a large creature's body. Cleaves closer to what is intended and gives you the chance to describe gross zombie stuff.

Quote from Fang_and_Lotus >>

In the text of Negative Energy Flood, it says that a target killed by the spell becomes a zombie at the start of your next turn. The spell does not limit this to humanoids, like Finger of Death. Would this mean that a DM might have to come up with a zombie variant for creatures on the fly? For example, let’s say a spellcaster killed a Androsphinx with Negative Energy Flood. Would the DM have to quickly come up with a zombie variant of its stat block, since it wouldn’t make much sense for the Androsphinx to become a humanoid zombie?

Negative Energy Flood specifically says to use the zombie statblock in the Monster Manual, which is presumably why the DM doesn't have to solve unsolvable problems, like figuring out what it even means to make a zombie out of a ball bearing that's been animated with Animate Objects.