Koi Fish

import { createAsset } from 'https://gaitkit.com/cdn/koi-fish.mjs';

const koiFish = createAsset();
scene.add(koiFish);                      // rests on y = 0, front faces +Z

let dist = 0;
renderer.setAnimationLoop(() => {
  dist += speed * clock.getDelta();
  koiFish.position.set(0, groundY + koiFish.userData.altitude, dist);
  koiFish.userData.tick(dist);            // the body waves with the distance
  renderer.render(scene, camera);
});

import { useMemo, useRef } from 'react';
import { useFrame } from '@react-three/fiber';
import { createAsset } from 'https://gaitkit.com/cdn/koi-fish.mjs';

export function KoiFish({ speed = 1 }) {
  const model = useMemo(() => createAsset(), []);
  const dist = useRef(0);
  useFrame((_, dt) => {
    dist.current += speed * dt;
    model.position.set(0, groundY + model.userData.altitude, dist.current);
    model.userData.tick(dist.current);            // the body waves with the distance
  });
  return <primitive object={model} />;
}

# Import koi-fish.glb (download below). It carries an in-place "Loop" animation.
@onready var anim: AnimationPlayer = $KoiFish/AnimationPlayer

func _physics_process(delta):
    velocity = transform.basis.z * speed
    move_and_slide()
    anim.play("Loop")
    anim.speed_scale = speed / 0.42   # one Loop = 0.42 m of travel

// Import koi-fish.glb with glTFast (com.unity.cloud.gltfast).
animator.Play("Loop");
animator.speed = speed / 0.42   // one Loop = 0.42 m of travel