What you are looking for can be done very easily nowadays. First you need a texture in the shape that you are looking for. You can shape a texture by adjusting the alpha layer of the TGA file.
Next you write an addon that integrates that graphic. A working health bar is this one:
Lua Code:
local function OnUnitHealthFrequent(bar)
local hcur, hmax = UnitHealth(bar.unit), UnitHealthMax(bar.unit)
local hper = 0
if hmax > 0 then hper = hcur/hmax end
bar:SetValue(hper)
end
local bar = CreateFrame("StatusBar","MyAddonMyHealthBar", UIParent)
bar.unit = "player"
bar:SetSize(100,100)
bar:SetPoint("CENTER",0,0)
bar:SetMinMaxValues(0, 1)
bar:SetStatusBarTexture(PATH_TO_YOUR_SHAPED_TEXTURE)
bar:SetStatusBarColor(1,0,0)
bar:SetOrientation("VERTICAL")
bar:RegisterUnitEvent("UNIT_HEALTH_FREQUENT", bar.unit)
bar:RegisterEvent("PLAYER_LOGIN")
bar:SetScript("OnEvent", OnUnitHealthFrequent)