The issues with the compression fitting have been addressed.
The brass flange fitting, the rubber gasket, and the threaded pipe is the "toilet spud". Replacements are available if the rubber is perished or the metal parts somehow damaged.
The flange nut looks pretty well bonded to the threaded portion. Soak well with WD-40 and let it soak in. There are splines inside the spud on which one would apply an internal spud wrench to keep the threaded pipe part from turning. You may find some other chunk of metal that will wedge against the splines, which in turn can be held by a conventional wrench. Holding the spud in place, remove the flange nut with a suitable tool.
Once the flange nut is removed, push the threaded pipe part inwards, but ensure it will not fall into the toilet. The part inside the toilet is conical shaped. Pushing it inwards will give enough free play that the rubber gasket can be pulled out. Once the rubber is out, there will be enough clearance in the toilet hole that the threaded pipe part can now be removed.
Examine the parts and evaluate if replacement is necessary. If the rubber is not too far gone, it may be possible to clean it, treat it with some sort of rubber treatment, and reinstall. If the rubber is cracked, hardened, rotted, or brittle, it will need to be replaced. The nut should be tightened firmly enough to ensure the rubber seals well against the toilet, but not so tight the rubber gets pushed out of position or the toilet gets cracked or broken.
You must have a vent. The toilet will not flush properly without one, and there will rarely be enough water left in the bowl. You need a 2" vent to take off from the branch line within 6 feet of the toilet. It can run up any convenient wall until it is above the toilet's flood level. Then it can run near horizontally (1/4" per foot) anywhere to tie into an equal or larger vent.
There's any number of ways to properly vent a toilet, and several common ways that are technically wrong, and even more that are just plain wrong. It's impossible to suggest a good method without a lot more information.
Best Answer
It's probably not an issue. There's a very small chance that it will get caught on a fitting and cause a clog, but more likely it'll either settle somewhere forever or make its way to the septic or municipal sewer. If it does clog a simple auger job should clear it.
Years ago I had my drain augered and the guy dropped a full size Vice-Grip down my aging 3" cast-iron pipe system. He tried for half an hour to retrieve it (magnets not being an option) but was unable. I figured it would come back to haunt me for sure, but it never did.