The SpiderPig is a special mob added in this mod. The SpiderPig, as it's name suggests, is a mixture of pig and spider. The SpiderPig does not, as of beta mod version 1.18, generate naturally in the world. However, there are plans in place to add natural generation in a later release.
Currently the only way to obtain a SpiderPig, is to craft the spawn egg itself. The spawn egg is crafted by putting 2 pig DNA and 2 spider DNA together in the crafting table. This will give you 4 SpiderPig DNA. To make the actual spawn egg, just put four SpiderPig DNA in the crafting grid.
In the future the SpiderPig will have better drops, and will spawn naturally in the map.