Best approach to use textures from a pool of thousands without loading all of them at the same time.

I'm having problems trying to come with the best approach to use thousands of textures in my game.

• I have 100.000 textures in total, each one of them is a pre-rendered frame of animation of an enemy (there are 100 types of them, so each enemy has 1.000 frames of animation).

• Each one of those textures is 256x256.

• There is going to be at most 100 enemies on screen, so I only need to load 100 textures at most at a time.

• If an enemy changes its current frame of animation, I need to discard his actual texture and replace it by another one.

• All 100.000 textures need to be always available to load since all 100 types of enemies can be seen in the same room during various points in the game.

• Atlases aren't useful because the textures are too big.

Is there anyway to dynamically change the textures without loading all of them into memory first? Keep in mind that I can't use the Resources folder because the textures are too big and numerous. I have Unity Free btw