Unmount for Rumrusher's mount here: https://gist.github.com/Rumrusher/6903877 Very, very adventurer-specific

unMount.lua

function unMount (rider,horse)
  --for k,horse in pairs(unit_list) do
        --if unit_rider.pos.x==horse.pos.x and unit_rider.pos.y==horse.pos.y
        --and unit_rider.pos.z==horse.pos.z then --check if they are on the same tile
        for i=#horse.general_refs-1,0,-1 do
            if df.general_ref_unit_riderst:is_instance(horse.general_refs[i]) then horse.general_refs:erase(i) end
        end
        rider.relations.rider_mount_id=-1
        rider.flags1.rider=false
        horse.flags1.ridden=false
        for i=#df.global.world.units.other.ANY_RIDER-1,0,-1 do
            if df.global.world.units.other.ANY_RIDER[i]==rider then df.global.world.units.other.ANY_RIDER:erase(i) end
        end
        return
end

function getRider(unit)
    for k,v in ipairs(horse.general_refs) do
       if df.general_ref_unit_riderst:is_instance(v) then return df.unit.find(v.unit_id) end 
    end
end

local selectedUnit=dfhack.gui.getSelectedUnit()

if not (selectedUnit.flags1.rider or selectedUnit.flags1.ridden) then
    qerror("Unit needs either to be riding something or to be ridden by something!")
end

local otherUnit=selectedUnit.flags1.rider and df.unit.find(rider.relations.rider_mount_id) or getRider(selectedUnit)

dfhack.with_suspend(unMount, selectedUnit, otherUnit)