We currently have lots of creatures that fill different roles except for a stealth creature. You could argue that a black moloch fills that niche but it’s simply too big to hide.
What if there was a new creature that you could only detect with passive sonar like the black moloch but was a swarm style creature?
Maybe they could latch on to the sub and act as sort of a hive mind to strike all at once instead of one at a time.
I’d call them Lurkers to keep on theme of the “er” names like thresher, crawler, raptor, etc