Many other users will try and scam your pokemon. There are many ways one can do so. some examples are
Scammer: hey @you can i borrow your aura mewtwo to defeat the elite four?
you: yeah, sure @scammer
seriously... don't be stupid... some other things people will do is to try and give you a glitched, hacked or duped pokemon that may or may not disappear when you trade for them.. to make sure they aren't check the OT (original trainer) if it is ???? check the date found. if it is also ???? check the stats if it has godly moves and stats then it may be glitched, hacked or duped