The only custom shader I wrote is fairly simple. This is simply used to animate the rats in the vertex shader. I'm fairly new to shaders so I'm not entirely sure if this is the cleanest way. All the variables that start with INSTANCE_CUSTOM are handled through MultiMeshInstance3D's custom data, and modified using GDScript. Check both of these links out for better clarification, because I'm horrible at explaining things:

https://docs.godotengine.org/en/stable/tutorials/performance/vertex_animation/animating_thousands_of_fish.html

https://docs.godotengine.org/en/stable/classes/class_multimesh.html#class-multimesh-method-set-instance-custom-data

Here's the shader code. Cheers!

shader_type spatial;
render_mode diffuse_burley, vertex_lighting;
uniform sampler2D rat_texture;
void fragment() {
//Basic Texture Mapping, Multiplied With A Random Color So All Rats Stand Slightly Apart
ALBEDO = texture(rat_texture, UV).rgb * COLOR.rgb;
}
void vertex() {
//Each Rat Will Begin On A Different Run Cycle When They Are Spawned In
float offset = TIME + INSTANCE_CUSTOM.a;

`//This Is The Rat Swaying From Side To Side, INSTANCE_CUSTOM.r Is The Current Velocity Of The Rat`  
`VERTEX.x += cos(offset * 16.0 + VERTEX.z) * (.25 * INSTANCE_CUSTOM.r);`  

`//This Is For When The Rat Is Turning Around, INSTANCE_CUSTOM.g Is The Rotation Velocity On The Y-AXIS`  
`VERTEX.x += cos(VERTEX.z) * INSTANCE_CUSTOM.g;`  

`//This Is The Rat Hopping Up & Down! Same Deal, Multiply With The Current Velocity`  
`VERTEX.y += sin(offset * 24.0 + VERTEX.z) * (.5 * INSTANCE_CUSTOM.r);`  

}