The Replicators
They pop up in both SG-1 and SGA; they are very different though similar.
In the beginning for SG-1 when the replicators were first introduced Colonel O’Neill was beamed up onto an Asgard ship by Thor (an ally) where O’Neill finds himself on a ship overrun with strange robotic and purple spider bugs. They are crawling all over the walls, ceiling, the floors, everything. As the bugs don’t seem interested in him O’Neill resumes his search for Thor and later finds his friend severely wounded and in a pod. Thor explains that the bugs on his ship are in fact Asgard's greatest enemy and that replicators consume any and all resources to create more of themselves. So if they ate steel the new replicators created by the queen would be made of steel. And that their plan is to invade Earth and strip it of its resources. O’Neill calls the SGC, General Hammond, Doctor Daniel Jackson, Teal’c, and Captain Samantha Carter (though I can’t remember if she had been promoted to Major at this point), and informs them of the situation. Requesting weapons like the M16/P90/rifles/shotguns and C4. Again I don’t remember exactly what guns they were but those are some of the most common weapons used. At this time the replicators could be destroyed by shooting them with bullets to blow them apart but it seemed that to wipe them completely they’d have to run the Asgard ship toward Earth so the ship could be ripped apart and burned in Earth’s atmosphere. For the episode only one replicator survived and caused many problems down the line but that isn’t the point of this post.
Now later down the line replicators learn the ability to create themselves into humanoid forms for both the Milky Way Galaxy and in the Pegasus Galaxy. The replicators of the Milky Way galaxy had been created by an android, a woman/robot with the mind of a literal child. She created the replicators as toys and said “toys” ended up destroying her original homeworld. Because she could create them she could control them. Daniel and his team hoped to study how she did this to stop the plague of replicators from getting any more out of hand. But she was later destroyed in the episode so the plan was (nearly) a bust. Now I don’t actually know how the replicators got into the other home galaxy of the Asgard or anywhere else for that matter but in Atlantis you also encounter replicators.
The replicators of the Pegaus galaxy (Atlantis) are already in humanoid forms and have a resentment toward their creators, the Ancients, and tend to see humans as their favorite child. So one can say that the Ancients had created the replicators and had possibly tried to create them in other areas of the Universe. Maybe in an attempt to keep the replicator's existence a secret from the Wraith who knows? Again this is from my memory and I don’t know everything.
Humanoid replicators from either galaxy have the ability to put their hands into people's heads and search the person's mind. They also have an insane amount of strength as they are technically robots. So don’t get punched or smacked. It's gonna hurt.
All three kinds of replicators from Pegaus to the Milky Way to spider and humanoid can be destroyed with an anti-replicator gun. The science of which I don’t recall and therefore will not get into but I think O’Neill created it when hyped up on Ancient knowledge.
Here is an image of the spider version of replicators.
And you know the humanoid version looks human but here is an image of the hand thing.