In my opinion, a pokémon fares best in its natural habitat. (Yes, I’m aware of what popular belief has to say about me. No, I do not keep the pokémon I add to my collection … of pokédex entries.) This may be especially true in your case, as it sounds like your goomy friend is a young one. Either that, or it was a runt hatchling. Regardless, it may be better off sent back to where it came from.
However, ultimately, it may be up to the goomy. The next time it arrives at your doorstep, let it know that you wish to help it find its mother. It will likely look at you quizzically (most pokémon can get the basic gist of human speech, but if it’s wandering onto your doorstep, the idea of a human helping it home may be unusual to it), but show it what you mean by leading it back to the edge of the forest. You can ensure that it follows you by bringing along a little bit of food, but it’s really better to allow it to decide whether or not it wants to go with you on its own, without an incentive.
If it does follow you, lead it back to the swamp. If you have pokémon that can help you navigate the terrain, now would be a good time to use them. Goomy tend to prefer the wettest, darkest parts of a swamp, so you may need to travel deep in before you begin finding anything that may resemble your friend’s home. Also be on the lookout for any signs of sliggoo or goodra activity. Sliggoo territories are defined by mucus-covered tree trunks, as sliggoo will often climb up trees to get at food sources. Goodra, meanwhile, can be found any place where there’s evidence of a burning by dragon fire … for somewhat obvious reasons. (Goodra need to train too, after all.) Trees that look like they were pushed over are also good indicators of goodra activity.
Once you begin seeing these signs, it’s very likely that you have reached the edge of your goomy friend’s home. Check its reaction, however, to be absolutely sure. If it looks like it recognizes the place (by growing excited or by wandering away from you), then that’s that. You may leave your goomy friend there and return home. It’s recommended that you do so quickly, as while goodra are not known for being particularly aggressive pokémon, they are still dragons, and you are still in their territory.
If, however, the goomy looks scared, turn back around and leave the swamp or go to a different area. You may have wandered into a rival goomy rout’s territory, or your friend may not want to return home at all. Keep a close eye on your friend’s reactions as you travel to see if it gets more comfortable the further away from the swamp it gets. If it doesn’t seem happy as you leave, keep looking in the swamp for another spot full of signs of sliggoo and goodra activity.
Alternatively, if the goomy doesn’t follow you to the forest or if it’s obvious that the goomy, once in the forest, doesn’t want to be there, you might be able to take it in. However, before catching it, be absolutely sure that this is what it wants. Open your door to it—literally. If your goomy friend crawls into your house or looks excited by the idea of being in your home, catch it. Goomy are rather easy to take care of afterwards. Just give them a nice, wet spot to live, preferably a tank or pool with plenty of clean water, a place for it to crawl ashore, and enough vegetables every day to keep it fed. Some goomy like to have large rocks in their habitats that they practice their attacks on, so this may be a suitable addition to your new goomy’s living space. (If it doesn’t spar with such a rock, at the very least, it will give the goomy another interesting thing to climb.)
Best of luck with whatever happens, anonymous!