Welcome. This is my first guide and as such it might seem a little bare. Alistar has been my fallback tanky support for a long time now and when paired at bottom with a decent carry we can almost always dominate the lane and snowball to carry the rest of the game. The top guides for Alistar all seem to treat him as a healbot with harass when in reality he can become an absolute unkillable tank with enough cc to feed your carries nonstop kills. I'm not some high level ranked player where strategies change as the level of teamwork and counter strategies rise but if you are at that level I doubt you'll be reading my guide.

Again a key point here though is you are NOT a carry. You can produce amazing sustain and cc to fill in a support roll, and be the tank that charges into the middle of them and throws them around like dolls, but without a good carry there to pick them off it doesn't work, so don't try to solo with this build.

This build doesn't rely on any specific runes. Probably typical Armor, MR, and CD or MS. MS is very important on Ali, cd helps you be able to keep being useful during fights that last longer than a couple seconds.

Masteries I go full tank with a dash of support for movement speed and mana regen, and cd reductions from offensive.

As a support tank it's all about teamwork. You need at least one decent carry to feed. In order to feed them you need to be smart about when to use your q+w combo. You can use it to either charge TO the enemy and stun him, or if you can make it to the enemy- headbutt him to your teammates then use q to stun him as he tries to run away. Of course take into account the types of champs, you don't want to headbutt a melee to your squishy ranged buddy, and pretty much never headbutt amu or anyone with a dangerous aoe INTO your team (headbutting them away as they try to ult can save them though).

Quick tips:
-Trying to catch up to an enemy to stun him for your carry? You can either flash on top of someone and headbutt them back, if you headbutt someone sideways into the terrain you can then q them and slow them, even if the terrain is ahead of them. There a minion or another champ near them? Headbutt to the minion and then q the champ.

-Being chased by someone? You can headbutt them over terrain so if you wait for them to catchup next to a wall (like in river), then headbutt them into it. A ranged chasing you? Stop right as you enter a bush so they run into you, then headbutt them away to give yourself more distance to escape.