I also didn’t like Ben’s original solution, as someone had said it was more a chance to spawn every amount of seconds than spawn every passage of time. When I was coming with an alternative solution I came to a conclusion that each type of villain would need its separate timer that would reset after spawning an instance of him, but keep timer of other villains going. To seenEveryXSeconds I added a small random range value to prevent form too much of regularity in distances between assailants. I also randomized lanes on which they are being spawned like someone before me. I very much like the result, more so that it isn’t overly complicated, rather simple idea. I also want to thank others for posting here.
There are a couple of unnecessary Debug.Log I needed to add for myself. Enjoy 
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class Spawner : MonoBehaviour {
public GameObject[] attackerPrefabArray;
public GameObject[] spawnerLines;
private GameObject parent;
private float elapsedTime = 0f;
private float foxTimer = 0f;
private float lizardTimer = 0f;
void Start () {
parent = GameObject.Find ("Attackers");
if (!parent) {
parent = new GameObject ("Attackers");
}
}
void Update () {
foxTimer += Time.deltaTime;
lizardTimer += Time.deltaTime;
foreach (GameObject thisAttacker in attackerPrefabArray) {
Debug.Log (thisAttacker);
Debug.Log (elapsedTime);
if (isTimeToSpawn (thisAttacker)) {
Spawn (thisAttacker);
if (thisAttacker.GetComponent<Lizard> ()) {
lizardTimer = 0f;
} else if (thisAttacker.GetComponent<Fox> ()) {
foxTimer = 0f;
}
}
}
}
bool isTimeToSpawn (GameObject attackerGameObject) {
Attacker attacker = attackerGameObject.GetComponent<Attacker> ();
Debug.Log ("Checking for bool");
if (attackerGameObject.GetComponent<Fox>()) {
Debug.Log ("Fox component found");
elapsedTime = foxTimer;
} else if (attackerGameObject.GetComponent<Lizard> ()) {
Debug.Log ("Lizard component found");
elapsedTime = lizardTimer;
}
if (elapsedTime > attacker.seenEverySeconds + Random.Range(0f,2f)) {
return true;
}
return false;
}
void Spawn (GameObject myGameObject) {
GameObject myAttacker = Instantiate (myGameObject) as GameObject;
int i = Random.Range (0,5);
myAttacker.transform.parent = spawnerLines [i].transform;
myAttacker.transform.position = spawnerLines[i].transform.position;
}
}