Would Hippowodon be a viable defender?
With a good pure ground typing, it only takes SE damage from Kyogre (and the upcoming HC Swampert) for notable generalists, and it resists Rampardos' best moveset. (As well as SD Tyranitar and a future Rock Wrecker Rhyperior) Fire Fang hits back against Ice and Grass types, (and Metagross to an extent) Stone Edge would hit like a truck with it's hard-to-dodge capability against Gyarados, Dragonite, Moltres (Double weakness!) Salamence and Rayquaza, and Ice types, despite not having STAB. (although it's resisted by Machamp) and Body Spam can be... spammed against the aforementioned Machamp, Mewtwo, Palkia, and other generalists. Not to mention it's Stamina x Def make it really bulky, and a good attack stat of 201 allows it to make use of it's pretty good defensive movepool.
Answers
Which doesn't change the fact that Blissey is the only mon which can effectively deter weaker players (or even time-pressured veterans) from attacking a gym.
Waterfall gyarados/milotic hits like a truck on defense, but that doesn't stop me from attacking a 3k gyarados/milotic with a 2k raikou or 2k FP meganium all day long.
Meteor mash metagross has the second highest TDO among gym defenders... but my 2k entei still melts a 3k metagross no sweat.
Hippodown may be an option as a defender if the gym you're chucking it in already has a blissey in it... but that doesn't make hippodown a GOOD defender by any metric. It's definitely above average, but above average is still just a speedbump.
Most "good" defenders are speedbumps. Milotic and Vaporeon can be sidestepped by Machamp. Metagross, without a resistance fares just as badly. Same for the Hippo. In fact, everything but Blissey/Lugia (Which can't be put in gyms) can be eaten alive by Machamp. (Even Drifblim/Gardevoir/Togekiss) The on
Also takes super effective damage from mamoswine, the new best ice type and isnt weak to rock. Nonstab super effective fast moves dont do too much damage to pokemon that arent glassy
It could be a good defender, but its stil got nothing on blissey, It will make a serviceable defender, but gen 4 also has a lot of other defender possibilities as well. Likely after the dust settles 5 days after gen 4's fully out, well see who's the best