I'm not talking building AD on support like thresh or sona, I mean A Champion that as AD ability and is a support. someone like lee but not lee sin. There no AD ability support only tanks and AP, would love to see Champion that Support/fighter as AD ability to save or protract ADC or other champion.
Poppy is/was an AD support that was somewhat popular for a little bit at least.
Ashe is thrown around sometimes as a sort of ad support, what with the slows and vision she offers.
Sion support is a thing.
One reason you don't see many AD supports is because it means theres only physical damage down there, so it'd be easy to counter with armour runes instead of needing both armour/mr (or in some supports case, they can just build armour directly).
Another reason is because AD items are relatively expensive on a supports budget, so assuming the support was playing properly (not csing or ksing), they'd always feel relatively weak unless their ratios were insane, and then you'd likely have issues with that champion being played in a carry role where they actually do have gold to pour into AD.
The problem with (viable) AD supports is that they will all be feast or famine. If behind, they bring little to the table. If ahead, they can do massive sustained damage and carry.
AD champions generally require a laner's/jungler's income to not get heavily outscaled by everybody else. With melee champions in particular, being restricted in gold-income means having to sacrifice either DPS or the amount of Beef required to survive an all-in, which really hinders the player to the point where it isn't worth it.
Trundle can work as a support, but that has a lot more to do with the amount of utility/free stats available to him on top of goofy amounts of sustain. There are probably a couple others but I couldn't name them off the top of my head--they aren't picked regularly for a reason.
Poppy support was a thing for a while, before the last major nerfs to her.
there was once a 
was used for his passive on his e which gave bonus attack speed to adc, vision small but still q to lower armor of enemy, e-q combo and ult for cc a shield for himself that also slowes. for his poke with spear, w for stuns/engages and his ult for roaming into jungle or mid for ganks. but that has been long ago. i think maybe a 
as a ranged ad bruiser support might be something fun. you have a slow, damage reduction to make you tank, armor reduction to make your ad seem stronger and a displacement as an ult. jsut a thought
maybe an support build be like a 
Once upon a time there was Gangplank Support. Also, Ashe Support. Varus Support (don't laugh, I've seen it tried... failed, but tried.)
Ashe?
I like doing 
support or 
. i get laughed at alot but ive made it work. plus it adds soooo much extra damage in the bot lane. =) just my thoughts. S5 and having fun doing what i want.
