I have 15 gal aquarium housing a Betta, an apple snail, and (as of today) the small specimen pictured below:
I temporarily quarantined the interloper in a small fish bowl pending identification. With help from this forum, I have concluded that it is a Pouch snail, but if a more experienced aquarist would confirm or correct this, I'd appreciate it.
The majority opinion seems to be that Pouch snails are the least desirable of the three most common uninvited species (behind Malasian Trumpet and Ramshorn), but that they are more helpful than harmful if their numbers are kept in check. Because I don't overfeed the Betta and I do weekly water changes, I'm thinking I should place him back in the aquarium and allow him to eat dead/dying plant matter, algae, and whatever else he finds palatable.
That having been said, the poop he took in the fish bowl suggests he's been getting plenty of food, so maybe there's more potential for infestation than I'd like to believe. Any thoughts/corrections/opinions/advice would be much appreciated!