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It was the penultimate hurrah to a huge rng project I did. It required emulator trading (a pain in the dick), chain breeding (Altaria's DD to Charmander then to Larvitar), and RNGing a shiny, 31/31/31/31/31/31 Larvitar. Of course, the finale was EVing it to outspeed the final boss in Battle Pyramid after one DD.
Normally you can just assume that their avatar is just something they like that they are using to express themselves. I guess the confusing part about your signature is there's no way to tell you took the picture yourself, so anyone looking could think you just googled "evolving into Tyranitar" and put it there.