From Melvor Idle
This page is up to date (v1.2.2).
Pet ID: melvorF:Otto
Source: Dark Waters
Drop Chance: 1 in 7,500 (0.01%)
Effect: -0.1s Attack Interval
Part of 100% Completion: Yes

Otto is a pet acquired at a 1 in 7,500 chance for each monster killed in Dark Waters.

Having Otto with you as a pet reduces the time between attacks by 0.1 seconds.