How to create a 3D model as mold for a 4-legged animal with parallel legs
In traditional mold making, this would require a &quot;sectional&quot; mold -- one that breaks into mutiple pieces to free the legs, rather than just two halves.  Sometimes you can use a non-planar mold break line to avoid this issue; look at the molds made for casting miniatures that aren't neatly arranged to see examples, but as you note, with one rear leg alongside the other, the narrow space between isn't accessible with a rigid mold and a molded figure that can't be flexed.
One way around this would be to print the figure and make a flexible mold from it -- the mold wouldn't be printed, but would be molded from a positive figure.  That would let you use a flexible mold material to run a join between the rear legs and still separate the mold, with a little care.  You may also want to make the mold in layers, as would be done for denture making -- using a soft, highly flexible material such as the alginate used in denture making for direct contact with the original figure, and then reinforcing the very soft contact layer with a sturdier backup layer to provide support.