Animal prints such as zebra and leopard can be difficult to draw directly on a cake with a piping bag, but piping gel makes it much easier because you can transfer a design on the cake. With this method, you don't need any artistic ability at all. Simply find an image of animal print that you like and copy it just as you would trace onto paper with a pen. There are some limits to what details will translate to frosting, so choose a large-scale image without any fine details.
Frost the cake with a crusting buttercream frosting made with vegetable shortening instead of butter. Apply a relatively thin coat but do make it thick enough to completely cover the crumbs and edges. Set the cake aside uncovered on the counter or in the refrigerator to develop a crust for at least 30 minutes while you prepare the animal print design. The frosting should be tinted the base color for the animal print, such as orange for tiger print or golden brown for leopard print.
Adjust your animal print image with photo editing software to change the scale as needed. Play with the scale so you can trace it easily and so the image will fit well on your cake.
Lay the image on a flat work surface such as a table or counter. Tear off a piece of parchment paper and lay it over the animal print image. If you can't see the lines of the image clearly through the paper, remove the paper and trace over the lines with a black marker.
Tape the animal print image to the flat work surface. Draw a circle, square or rectangle on the piece of parchment paper to the same size as the top of the cake. This saves you from tracing more of the design than is actually needed. Do the same with the sides of the cake. Tape the parchment paper in place over the animal print image.
Insert a round decorating tip in a decorating bag. The size of the round decorating tip determines the thickness of the lines. Put clear piping gel into a small mixing bowl and add gel food coloring to tint it to match the colors in the animal print. Use black tinting gel for zebra, tiger print or leopard print. While you can transfer a design with clear piping gel, it's much easier to see the tinted lines.
Trace over the lines to apply the image on the parchment paper with piping gel, starting from the center of the paper and working toward the edges to prevent smudging. Squeeze the bag with firm, constant pressure to create a steady flow of piping gel from the decorating tip. Stop squeezing the bag when you complete a line. Lift straight up away from the paper to break the stream of piping gel and reposition it at the beginning of the next line in the animal print pattern.
Remove the tape and lift up the parchment paper carefully. Flip the paper over and set it in place on top of the cake. The cake should have only a thin, crumb coat of buttercream frosting when you transfer the design. Trace over the lines lightly with a small artists' paintbrush to press the piping gel design into the crusted buttercream frosting.
Peel the parchment paper away from the cake slowly, leaving the animal print transferred on the cake. Pipe more piping gel on the cake where needed to fill in any lines that didn't transfer well.
Fill in the piping gel lines with buttercream frosting tinted with gel food coloring to match the animal print, such as black to fill in tiger and zebra stripes, or brown to fill in leopard spots. The first option, and perhaps the best option for small spots, is filling in the lines with a dots made with a small star decorating tip. Alternatively, you can fill in the lines with a round decorating tip and spread the frosting smooth with a small icing spatula.