Random Malaysian Trumpet Snail...I Think.

Looks like a young MTS to me. Very beneficial to the aquarium as they bury themselves and turnover the substrate, preventing gas build up and eating detritus. However, overfeed your fish and you'll have a population explosion. They give birth to live young and if there was a snail on the cave, there may well be eggs too. They're very resilient little creatures, so I'm not surprised this little fella survived. If numbers start to increase quickly, cut back on your feeding. I'd also recommend a couple of assassin snails to keep the numbers down.

Some information for you link

What it doesn't say is that these snails can breed by parthenogenesis - they don't need two of them, one can  breed quite easily by itself.

I got that from Tropical Fish Hobbyist Magazine

They have a third and very descriptive name as well—the Malaysian livebearing snail. These snails can reproduce both sexually and through parthenogenesis, starting at a size as small as 10 millimeters! Instead of eggs, these snails give birth to as many as 70 live young at a time. With a reproductive strategy like this, you might guess that they can increase in numbers rapidly. And you’d be right.