Clothing
Pokemon Video Game Nationals T-Shirt

Granted it's only a silhouette of Mewtwo, but it's still worthy of inclusion. I believe this is back from when Mewtwo was still tournament legal in Pokemon video game championships - something I hope Nintendo goes back to in future years.
Pokemon 151 T-Shirt (Japan only)

This is one of the nicest official Mewtwo shirts out there, and I believe may be the rarest. This was the first attempt to have Pokemon shirts geared specifically towards older players - and if the shirts weren't the equivalent of $45 each and were made with larger people in mind, I maintain it would have worked. I still wear it for special occasions. I haven't even seen one of these pop up on Y!J in a long time, let alone eBay. If you see one, snatch it up ASAP!
Pokemon World Championships 2006 T-Shirt, Purple (Competitor)

These were only given to those who played in Worlds 2006 - I was lucky to snag a couple of these, as for a while the purple shirts seemed to be the hardest to find on the secondary market.
Pokemon WC2006 T-Shirt, Yellow (Volunteer)

This one was, and I believe still is, much more common - they most likely had a good number of volunteers and gave these to anyone who may have helped out with the event. Given Mewtwo is partially purple, I still prefer that version but those looking to just get one of these shirts will have much more luck finding them at a reasonable price.
Pokemon WC2006 T-Shirt, Black

This is the only black shirt I've ever seen. I know I'm still missing the red one (Translator) - but oddly, this one doesn't have a label on the back as to who it was for. Was this for staff? Champions? I have no clue. Maybe someone who sees this can let me know.
"Accept the Challenge" T-shirt

This isn't anywhere near my favorites; I remember picking it up really cheap back when I was collecting anything and everything that had Mewtwo in that pose. I think I've seen a few of them pop up here and there since then.
"Training is Over" #1

Nintendo really seemed to like this line, throwing it on multiple items. I'm guessing this one came out right before the movie did, while they were still being mysterious with some of the details.
"Training is Over" #2

Okay...that pose for Mewtwo is just freaky. And makes him look like he doesn't have any teeth.
Toddler Shirt

If they were going to throw as many poses as they possibly could on one t-shirt, at least they went with the good one enlarged. Arceus willing, we might even see a little one wearing this someday.
Random shirt

Not much more to say other than some interesting shading and the standard pose.
"The Challenge is Greater Than Ever" T-Shirt

One of the few decent shirts, besides the Worlds shirts, that'll fit most adults.
Kids' Swimming Trunks

Tiny and frail as they are, it was a Mewtwo item, I was obligated to pick it up.
Mewtwo/Mew Socks (from Japan)

For as many T-shirts as we've seen, there was very little in other categories of clothing. And for how small these are, they did a damn good job making Mewtwo look good on these.
Mewtwo Socks (New!)

They did a much better job with these - but that's probably me being biased because Mewtwo is so much more prominent.
Pokemon WC 2006 Hat - Back

Yet another batch of well-done World Championship stuff - shame Mewtwo's only on the back of this one though.
Pokemon WC 2006 Hat #2 - Front

Seems they did two different colors of this but swapped the positions of the logo and the pose on it. Needless to say I wear this one more often.
These were rare for a while - fellow Mewtwo collector Honestboris picked up a bunch of these on the cheap and traded me one a long time ago - it was actually what brought us back into contact after a good couple years.
These were rare for a while - fellow Mewtwo collector Honestboris picked up a bunch of these on the cheap and traded me one a long time ago - it was actually what brought us back into contact after a good couple years.
Pokemon WC 2006 Hat #2 - Back

Just to show the logo, and that it's a legit WC2006 item.
Pokemon Organized Play Hat (Standard)

This is another hat I quite like the design of. Much easier to get than the WC ones. This is the common one though - there's a much rarer one with "STAFF" embroidered on the opposite side of the POP logo. I've only ever seen one - it's in Honesboris's collection.
Sears Hat

All I remember about this hat is that either this one, or the other variant below, were Sears-exclusive, along with a Charizard hat done in the same style.
OLD Sears Hat

This one seems to be much rarer - without the #150 in the corner. I had accidentally let my first pickup of this hat go through the washer not realizing it wasn't machine-safe, but since obtained a replacement in good shape!
Kids' Hat

I don't think they could have made Mewtwo's name any uglier on this hat if they tried, nor could they have gotten the color scheme any more wrong. Oh well, there's always the WC hats for actual use.
Mewtwo beanie

And now we've got one to wear in the cold weather! Lots of these type of items seemed to come from eBay Germany - good luck getting a hold of anything from there if you're a US buyer.
Youth Hat - Blue

Not much to say here - we're long past any of the Mewtwo hats I actually wear.
Youth Hat - Black

Mainly including for completion's sake.
Mewtwo Beanie #2

One more from eBay Germany.
Neckleash

These lanyards are durable, cheap, and one guy on eBay was selling these for like $1.50 each. I ended up picking up at least 48 of 'em at the time and giving them out to friends at conventions and such. (If you are a Mewtwo fan and happen to need to display a badge for any reason, these are the perfect item to do it with.)
Crappy plastic rings

These seriously look like knock-offs. But I see them enough to think they're just really, really crappy legitimate merchandise.
Mewtwo towel

This one happened to be a heartfelt gift from Cheshire-Kat, the Mewtwo fanfiction authoress. She ended up sending me a good few things that I hadn't encountered before - most of which I've since picked up dupes of. I think she said she picked it up in Wal-Mart way back when. I still use this when going to a water park.
Mewtwo towel #2

This towel I've only ever seen once, and from what I remember was a lucky cheap snag on eBay, like ~$10 or so. If you see one, pick it up ASAP.
Banpresto Giant Mewtwo/Mew Handkerchief

This one came over from Japan - of all the items in my collection I use on a regular basis, this is NOT one of them. It would feel sacreligious somehow.
Giant Handkerchief #2

One more from the Banpresto line.
Mewtwo Armband

More Movie 16 stuff.
Mewtwo Wallet

My only complaint about this wallet - aside from the pose - is that there aren't many slots inside to put credit cards in. But then again, that would make sense since the makers of this wallet figured little kids don't really need credit cards.
Mewtwo/Pikachu/Meowth Tie (RARE)

Another Japanese find - if I weren't so lazy as to use clip-on ties most of the time I'd be wearing this at work on a regular basis. I wish there was one that was more Mewtwo-centric but I'll take what I can get.
Mewtwo Sunglasses (front view)

These things are quite tiny - they won't fit most Mewtwo fans, but given how difficult they were to find they're still a nice trinket to have. They don't feature Mewtwo much though - just a little picture of him on the side...
Mewtwo Sunglasses (side view)

...as seen here.