Hello. I just found this plug laying right next to this hole. Dont know what either are and just plugged it back in. Can anyone tell me what it is?

 

The only thing on my engine that looks exactly like that rubber plug would be my fill hole for the automatic transmission. My plug has a dipstick on one side and a handle on the other. Is your car an automatic or a standard? It's more than likely a fill plug for your tranny.
Duane