There are a lot of Pokemon that would make decent partners to Miltank. Choice Scarf Rotom-S is a good one. I'll just list a few that I think would semi-work:
Amoonguss: Defensive 'mon with Spore, weak to Fire and Ice but resists Fighting like a champ. Good partner for Thick Fat cows.
Golbat: Defensive 'mon with a variety of support mvoes, 4x Resists Fighting moves aimed at Miltank, weak to Ice moves that cow can sponge with Thick Fat.
Bug/Poison and Bug/Flying types in general, although most of them suck.
Drifblim: Decent bulk, nullifies Fighting moves, likes cleric support.
Garbodor: Sets up Spikes, Toxic Spikes, has decent bulk, resists Fighting moves.
Bulky Psychic types: Fighting resist, usually have many good support moves, draw in ghosts that Scrappy-Tank can waste (I guess). We're talking things like Musharna, Defensive Mesprit, Eviolite Duosion, etc.
Ninjask: Ever seen a Miltank with +3 Spe and +4 Atk? You don't want to. But really that just goes for any bulky 'mon that Ninjask could pass to in the first place.
Rotom-S: Choice Scarfed variants are some of the best Pokemon in the current metagame, and generally work well with any team. Rotom-S has the nifty ability to come in on any Fighting attack aimed at Miltank and lay waste to whatever poor fighting 'mon decides to stay in on it. It also appreciates cleric support, can trick away its scarf to cripple things that might try to use Miltank as set-up fodder, etc. Really it's more that Rotom-S is a good Pokemon than it's specifically good for Miltank, but whatever.
There's probably more, and I'm probably wrong about a few of these, but those are the ones I think of. Anything look good out of that?
EDIT: merlinnerd922, tell that to Gorebyss, Swellow, Sawk, Braviary, Jynx, Rotom-S, etc etc etc. I fucking miss Magneton and Lanturn >:(
No Luck Involved: Gorebyss isn't broken, it's just the best set-up Pokemon in the tier. I think a decent amount of people complaining about it at this point just don't run good enough counters/checks to it. It CAN be played around, it CAN be defeated. It's just a pain in the ass to stop once it's smashed.
DOUBLE EDIT: Just realized how repetitive this post was with all the fighting resists I mentioned, sorry about that.
HEY GUIZ, DID U KNOW THAT POGEYMANZ DAT RESIST FIGHTING CAN HELP NORMAL TYPES???