1 Creeper and 1 Skeleton (almost) always present on the Nether side of a Nether Portal

Nearly every time I teleport to the Nether using a specific Nether Portal, there's a Creeper and a Skeleton standing in/immediately next to the portal on the Nether side. The Creeper explodes instantly when I arrive, or slightly before, killing the Skeleton, hurting me, and leaving a worrisome hole. I'm not sure how Overworld mobs are meant to behave around Nether Portals, so I built a fence around it on the Overworld side, and a building around it on the Nether side, and covered both in torches, but I haven't managed to stop the mobs from being there when I arrive.