From косат (kosat, “hatchet, fin”) (dated) +‎ -ка (-ka), a calque of Latin orca (originally meaning oryx, pickaxe).

• IPA^(key): [koˈsatkɐ]

коса́тка • (kosátka) f

1. grampus, killer whale, orca

• “косатка”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Sofia: Bulgarian Academy of Sciences, 2014
• “косатка”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Chitanka, 2010

Russian Wikipedia has an article on:

косатка

Wikipedia ^ru

From коса́т (kosát, “oryx”) (dated) +‎ -ка (-ka). By surface analysis, коса́ (kosá, “scythe”) +‎ -а́тка (-átka). A calque of Latin orca.

• IPA^(key): [kɐˈsatkə]

• Audio:

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коса́тка • (kosátka) f anim (genitive коса́тки, nominative plural коса́тки, genitive plural коса́ток)

1. killer whale, orca (Orcinus orca)
2. falcated duck (Anas falcata)
3. naked catfish, bagrid catfish (Bagridae spp., Siluriformes).
4. obsolete spelling of каса́тка (kasátka); barn swallow