Many Pokemon will evolve through trade. Four of them don't require a hold item, and the rest do. Kadabra evolves into Alakazam, Graveler evolves into Golem, Machoke evolves into Machamp, and Haunter evolves into Gengar when you trade. The rest are down here, or at least what I can think of:
Need to hold Metal Coat:
Need to hold King's Rock:
Need to hold Dragon Scale:
Need to hold UpGrade:
There are also Pokemon not obtainable in Crystal itself that must be traded from other versions to get. They are:
Keep in mind that in Red and Blue, you choose from only one of Bulbasaur, Charmander, and Squirtle, so you would need three of any of these games. However, in Yellow you can get all three, so it's probably the best choice. You also get a choice between Omanyte and Kabuto in all three games, so that will require you to get another. Mew and Celebi are only available via GameShark, unfortunately, so you'll have to make do without them unless you're okay with hacking your game.
In order to get all Pokemon, I would reccommend getting a Yellow version, and then either a Red or Blue version, (Or both!). You'll also need either Gold or Silver.
If you can trade Pokemon from crystal version to gold version my guess is yes
There is no NPC who you can trade a Haunter to for a Gengar in Pokemon Crystal. To get Gengar you will need to trade with another player or a copy of Pokemon Gold, Silver or Crystal.
It is not available in Pokemon Crystal. You will need to trade for it from Pokemon Red, Blue, or Yellow.
No.
trade it from red,blue, or yellowMewtwo is not catchable in the wild in Pokemon Crystal.
To trade Pokemon from Pokemon Crystal version to another version of Crystal you need to use a link cable. You can also battle this way.
If you can trade Pokemon from crystal version to gold version my guess is yes
There is no NPC who you can trade a Haunter to for a Gengar in Pokemon Crystal. To get Gengar you will need to trade with another player or a copy of Pokemon Gold, Silver or Crystal.
no
It is not available in Pokemon Crystal. You will need to trade for it from Pokemon Red, Blue, or Yellow.
No.
Yes but don't trade pokemon only from crystal to red for example don't trade a lugia in red or else ur game will get messed up but u can trade any other pokemon that were in red from ur crystal
trade it from red,blue, or yellowMewtwo is not catchable in the wild in Pokemon Crystal.
Trading between Crystal and XD is not possible.
Trade
You can't
Yes.