The two-headed dragon idea will be implemented as part of the grave drake. Additionally, the Tarasque will likely be implemented. The Garrgouille will require DFHack, meaning that I'm not going to deal with it right now until I get the first release out (which likely won't contain the Grave Drake or Tarasque either, just because those will be a little more difficult to mod).
I've sometimes thought that if the description of the Amphisbaena were of a true biological, it would probably be referring to a false-head, like a centipede's, or a pincer or thagomizer mounted on the tail. Maybe something to work with there.
Does the Gargouille really need DFHack? I think you can just specify what material the breath attack should use and make it water. I think it's [MATERIAL:WATER:BREATH_ATTACK] though the syntax may be off. MATERIAL can be any of the material tokens, AMBER, CORAL, GLASS_GREEN, GLASS_CLEAR, GLASS_CRYSTAL, WATER, COAL, POTASH, ASH, PEARLASH, LYE, MUD, VOMIT, SALT, FILTH_B, FILTH_Y, UNKNOWN_SUBSTANCE, GRIME. And of course, INORGANIC Magma when specified with subtype NONE or NO_MATGLOSS, otherwise a user-defined inorganic material. Which does bring up the totally gnarly idea of a dragon that "breaths" VOMIT. Frat Dragon. Ash, glass and rocks would also be interesting. There's also the beautiful UNDIRECTED_ITEM_CLOUD:ANVIL.