I believe that DAZ has centaurs for some of their figures. Also search at 

http://www.sharecg.com

There are some there - one for the DAZ Aiko3 figer for example. That one and probably the others at DAZ will require the DAZ Millenium Horse figure. They used to have a free l'lte ' version of the horse that would work fine but I'm not sure it's still available. If you search freestuff here, there is a centauress by Nerd3D made using the very old Poser 4 female (aka Posette) and the Poser 4 horse. I'm not sure if those are available for Debut.  Ditto mermaids search at DAZ and ShareCG.

 http://mystic-nights.com/directory/index.php  is a good search resource for free Poser items as well. Just remember to look carefully at which base figures are required to make the creatures e.g. Victoria 4 (aka V4), Aiko 3 (aka A3) etc.

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