So in short words we need to have an animation file (one that we could probably swap later), set it up in the Animator Window by adding a State, Parameter, and Transition, and finally in the Scene window, a Behaviour.

Making an Animation

I made a simple cube from Unity, gave it a scale of 1, 2, .1, and set it to zero. It's pretty important for animations that they are set to zero in Position. So that may require you to just give it a parent so the object is set to zero in relation to it.

While the object, in this case the cube, is selected, in the Animation tab (you can find it under Window>Animation>Animation if you don't see it) click Create in the center of the scrub window.

Once you name and save the .anim file anywhere you need, You now see what you need to edit the the movement. This can get really robust. This is very powerful, so for this, i'll keep it simple.

click Add Property

click the tiny right facing arrow next to Transform

click on the really really tiny + sign to the right of the transform type you want to animate (in our case - Rotate)

Once you do, the rotation.x, .y, and .z show up in a little hierarchy to the right of the timeline. if you don't see it, hit the teeny weenie arrow to the left of the game object. Everything is so tiny...

You can just type in the parameters you want for your rotation. If you select a place on the timeline, a keyframe will just be added, you can select keyframes, or you can hit the 'record' icon (red dot) and rotate the object to any free form position in the scene.

I want to rotate the Y axis -90 degrees to open 'away' from us. You might notice it changing it in the inspector too. It's fine if it's highlighted in blue. It means it's being animated.

Not to be confused with the Animation window, this window is completely different. It's a state machine. Like a visual hook up for transitions and parameters. This also is very powerful, and very robust. I highly recommend going down this rabbit hole if game animation is something you want to dive into. This system is also known as Mecanim in Unity speak.

When you go into the window after you have created an animation, it'll look like this with the Inspector Tab docked next to it (This will be relevant later):

Right click anywhere on the grid and you will see this. We want an Empty state.

This will create a node called New State. We can rename it, and we should. I'm going to call it "StartsClosed". You can do this in the Inspector tab if you have it selected. Just click where it says New State on the top and name it whatever you want.

You can't do this until we have another state created. Now that we have, right click on your new state, ours is StartsClosed, click make transition, and drag it to DoorOpen.

right-click StartsClosed and select “set as Layer Default State”

See how the arrow from entry moved from DoorOpen to StartsClosed? That means StartsClosed is now the initial state of our state machine.

Now we need to click on Parameters (the tiny tab next to layers) in the top Left part of the Animator tab, then the tiny plus sign to the right of the name field, then you'll see a drop down of some types of data. We want to select Trigger. Fun fact, a trigger is almost exactly the same as Bool, but has some additional logic attached to it. Just a Unity thing.

Now you'll want to give it a name. Remember the name, we are going to use it shortly. It is case sensitive, so for best practices, don't use spaces, numbers, or punctuation. Keep it simple. I'm calling the trigger "open". 

Click on the arrow between StartsClosed and DoorOpen in this example. This is called a state transition. Once you do it'll turn blue. and stuff shows up in the inspector. We are looking for the 'Conditions' box. It should say "List is Empty".

Click the tiny (+) sign. It will create a new condition and since we only have 1 parameter, it defaults to that one.

Okay, lets head back over to your scene, and create the trigger. Add the TriggerAnimatorOnEnter script to your object you want as your Trigger. Make sure the object you are creating has had the "Is Trigger" box checked in the collider of the object.

Then make sure the Trigger field has the name of your parameter that you made. In our case, it needs to say "open", still case sensitive.

Make sure your character (or the object entering the trigger) has a rigid body component added to it.

I renamed the Original box that we animated to be called Door_Object. Since we have 2 cubes now. Then under the Animator field inside the "Trigger Animator On Enter (Script)" component make sure that says the name of the object you animated. The now named Door_Object.

Should all look like this:

And you did it! You successfully set up the Animation Trigger!

Now, if you want to keep your animation from looping, just find the file in the project, and under the inspector tab and uncheck Loop Time. Should do the trick!