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Not too experienced in sigs, but here's what I've done so far. I've never actually learned how to make them or done tutorials are something, it's all just fiddling around.
After seeing this thread I was inspired to try this thing. Here's my attempts so far, first attempt to last. Criticism?
Houndoom + Gorebyss (tail) + Cyndaquil (fire)
Rayquaza + Wailord The part where they connect on Wailord feels....incomplete.
Poliwhirl + Deoxys