Are plastic bins good for pony storage?

[Mana Minori]

Maybe it depends on how perfect you want them to come back out?

It doesn't matter what you store - if you want something to come back out in the same packed away condition you have to take some measures to ensure that is the case.

Plastic is not the best option - unless you ventilate the box and store the ponies in acid free tissue paper. But then again, if its all you have, then you just have to make it as ideal as possible for your situation. The amount of time they will be stored, the atmosphere conditions of where you live are also large part of what you actually have to put in place.

As Baby Sugarberry says, you may encounter more issues with ponies causing issues with others stored in the same place than the plastic box itself.

If you want them to come back out as perfect as possible - wrap them in acid free tissue paper, ensure to wrap hair around legs and protect each by having tissue paper between hair/body. Ventilate the box and line it. Store the box in a place which is not subject to great differences in humidity/temperature and ensure to check on them (rotate them) regularly.

could I get the tissue at a craft store, like Michael's? And I need to tissue the hair separately from the pony? Or the whole thing in like one big tissue pony packet? So I seal off the tissue with something once the pony is inside, or let it breathe? Would poking holes in the plastic container work for ventilation? (dunno how that'll work, with no tools)

[Strawberry Swirl]

What if the ponies are put in ziploc bags before being placed inside the box? That is what I am currently doing anyway.

[banditpony]

What if the ponies are put in ziploc bags before being placed inside the box? That is what I am currently doing anyway.

Don't close them if you do. No air flow is going to accelerate deterioration (like stickiness of plastic break down. )

It's still better to use a acid free tissue paper...

[Artemesia's Garden]

I ordered some acid free tissue paper on eBay and cut it up into pieces, wrspping each pony with mane and tail in place. Not worth scrimping on this imo.

[Baby Sugarberry]

Okay, so the main issues with storing ponies in plastic are two-fold; two unstable materials (plastics) in contact with each other, and the creation of micro-climates.

Let's start with plastic.

Plastic is not a single homogeneous substance.  It's literally hundreds of thousands of different materials, many with wildly different properties.  Two plastic bins that look the same may very well be made from different types of plastic and won't behave identically even under similar conditions.  Thus it's a toss-up whenever you get a plastic bin how it's going to react (or not) to other materials and the environment you put it in.   Plastic isn't a stable material.  It changes as it ages, just like our vinyl ponies.  Lots of things are added to plastics to make them more flexible, a certain colour, fire-retardant, more stable, easier to fabricate, etc.  That mix separates and/or degrades over time, and we end up with yellowing, brittleness, spots, stickiness, shrinkage, etc. This is entropy, and it's unavoidable.  All you can do is slow it down.  The more fluctuations and adverse environmental factors you expose a material to, the faster it happens.  Which brings us to...

Microclimates.

A micro-climate is a localized condition in storage that has a different environment than the surrounding area.  Think a tupperware container in your fridge as an example.  Unless created deliberately, they're usually bad.  Ponies and other plastics in small, sealed areas create adverse microclimates as they break down.  Sometimes the thing you store them in or touching can also contribute negative factors to their environment.  When the macro-environment a sealed container is in fluctuates in humidity or temperature, this can create adverse conditions in the smaller container.  Humidity is relative.  The relative part relates to temperature and how much water air can hold.  A sealed micro-climate can't equilibrate its humidity as the temperature around it changes, and there's nowhere for any volatiles to go. 

What does this mean for storing collectibles?

Don't seal your toys in bins or bags.  If you must put them in an airtight container, ensure they're cycled or opened regularly.  Minimize contact between types of plastics.  Store away from light, water, extremes in temperature. 

could I get the tissue at a craft store, like Michael's? And I need to tissue the hair separately from the pony? Or the whole thing in like one big tissue pony packet? So I seal off the tissue with something once the pony is inside, or let it breathe? Would poking holes in the plastic container work for ventilation? (dunno how that'll work, with no tools)

The reason separating hair with tissue paper is sometimes recommended is because certain nylon colours, particularly neons, can bleed into vinyl, especially when pressed against each other long term as often happens in storage.  I haven't seen this with colours other than the neons personally.

Any acid-free tissue paper in white is fine. 
There's no need to seal the tissue paper, it's just a physical barrier against transfer.
Regularly opening your bins and checking on the contents will be more effective than attempting to poke holes in them.  The other alternative is leaving the lids off if storage space allows.

[WaterDraw]

Oh yeah, and also make sure your pony's hair is styled the way you want it to. I've had too many ponies end up having a split in their mohawk because of poor packing...
It's a minor detail, but important for future displaying!