The Lake Equivocolao can be found north of Port Hope and south-west of Banuta (here). This lake is the only place that Shimmer Swimmers can be found. To be able to catch some of them you need throw corpses into the water. The corpses can be from any creature, not necessarily humans.
See also: Twisted Waters
Its name may be a combination of many words and/or allusion to many brands (Equivo, Colao, Equivoco, Vocolao, etc.).
Tips: there is a perfect place right by the water where you can train so you only have 2 slimes on you at the same time. This is perfect if you are trying to get the Shimmer Swimmer so you can train skills at the same time.