There are millions of halos residing in the IllustrisTNG simulation, each of which has its own story to tell of how it came to be. Here we are showing the time evolution of the largest of these halos, which we affectionately call the "Pokemon", as by the final time step of the simulation, it has acquired a tail and two smaller substructures resembling a head and a body. It is remarkable to see how much structure gets accreted onto our "Pokemon", some of it appearing quite unphysical. For example, in the final step it is made of 3 very distinct halos which are identified as one by the FoF algorithm. A known issue of the FoF halo finder is that it tends to join together distinct structures which are in close proximity and this is clearly illustrated in this case.

Here we show what the final distribution of the particles in the largest halo looks like. As we zoom into one part of the halo, we notice that indeed it appears to be gravitationally bound and quite distinct from the rest of the halo structure. Furthermore, the overall shape of the halo does not seem to exhibit any of the expected hierarchical features of a dense core, in which the central subhalo resides, surrounded by a distribution of small satellite subhalos. In this case, two of the subhalos appear to be roughly equal in size, so neither of them can be clearly identified as the central.