Not sure why it shows as a modem on the config, but the Dynastream ANT2USB is driver used by the Garmin USB stick that is used by Garmin devices to download information to the Mac (I have a Garmin Forerunner 410 that uses it). It uses this ANT2 protocol created by Dynastream that works more or less like Bluetooth (I am not an expert, but that is what it looks like).
It seems suspicious to me.