This tutorial will inform you on the building of pig randomisers, a simple method which was once commonly used to add an aspect of randomness into redstone builds. They work using the fact that mobs randomly wander around, and by using pressure plates or tripwires this random wandering can be converted to a redstone signal. Builds which often contain pig randomisers include gambling machines and boss battles. (Please note that randomisation can now be done in a more compact way using droppers, hoppers and a redstone comparator)
It is recommended that you use passive mobs, as they won't despawn and will be easier to place inside.
If you do feel the need to use a hostile mob, either give it some armor or something to hold (this will work with zombies, skeletons and zombie pigmen) or use a name tag on it. All of these will prevent the mob from despawning.
If the mob is too far from a player, it will stop moving, so it is recommended that you place the randomiser as close to the player as possible.
Two commonly used mobs in pig randomisers are snow golems and chickens. Snow golems are used because they move silently and can be created inside the randomiser, and chickens because they are one of the smallest usable mobs (allowing the randomiser to be compact) and they can also be created inside of the randomiser.
To decrease how often the redstone is activated, make the room bigger or remove some pressure plates. To increase how often the redstone is activated, do the opposite.