The best replacement for mermaids is sirens.

The best alternatives to mermaids are rated as following:
★★★★★  sirens
★★  pirates
★  whales
★  manatee

Your alternative for mermaids is not listed? Suggest!
Speaking about mermaids, one probably shouldn't forget about beached whales.

Other search terms related to mermaids and their ratings are:
★★★★★ beached whales
★ mermen
You have in mind a companion for mermaids and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mermaids in all years. Click on graph for more.
mermaids
sirens
pirates