Here's another hero that doesn't look all that hot as a support. After writing this I might have to revisit my Bloodseeker post because the hero got a rework in 6.82... Based on the changelog he might actually be more potent as a support than before. Anyway, this post is about Bristleback.
This time we have two actives to work with: one deals accumulating physical area damage and the other accumulates slow and minus armor on a single target. Both of these skills interact with his ultimate which gives him free movement speed and damage whenever he casts one of his low cooldown spells. Since it's actually not all that common to build early mana pool items on Bristleback, we can safely assume he will have enough mana to fight even when played as a support. The bigger issue will be that his entire hero concept is built around surviving long enough in a fight to actually accumulate all that minus armor, area damage and bonus damage. His passive certainly helps, but only to some extent. Another problem is that as a support, you really need to get at least one level of Viscous Nasal Goo early because it's your only disable. And with only one level in it, the max slow is 32% and it takes 6 seconds to apply fully. You get the same slow with only two casts on level two. Most importantly, the minus armor doubles at level three.
All in all, it's not all that amazing as a level one disable. While being able to keep it up indefinitely is good for long chases, early game ganks happen so close to towers that being able to cast it more than once or twice is highly unlikely. With one cast the slow is 23% on level one. The cast range is 600, which is decent enough. However compared to any level 1 slow in the game it's quite obviously abysmal. The primary benefit is its theoretically infinite uptime. It's also easy enough to switch targets with its 1.5 second cooldown, although you need to start building the stack up from zero again. Quill Spray also accumulates its damage rather slowly. The problem I see here is that Bristle doesn't offer what you'd generally want from a support in the current meta: he cannot just come out of smoke and make a gank happen because he doesn't have a good instant disable or burst damage. Sure, there are supports who are not primarily gankers (e.g. Abaddon), but Bristle's ability as a defensive support is also vastly inferior to theirs.
Quill Spray does have some farming potential however, and it might be possible to run Bristle as a jungler. This is something I have to test, although I am kinda skeptical as his starting mana pool is not all that hot (his int growth is decent enough tho). Might as well do some math. On level one, five casts of Quill Spray costs 175 mana (of his 182) and does 20+50+80+110+140 - or 400 - physical damage. Most jungle creeps have 1 armor, which reduces this damage by 5.7% to roughly 377. This damage is enough to kill most small camps, and bring the medium satyr camp low enough for you to finish without too much trouble. The wolf camp is also brought very low. Problem is you need to kite and tank the camp for 15 seconds to cast all these (the small satyrs should die after 4 casts though). The satyr camp is ideal, and killing it gets you to level 2, adding Bristleback to your available abilities. However, there would be no mana left. In other words, jungling doesn't look very good on paper.
To summarize, in theory support Bristleback can do just as much damage as a core with his abilities. However, the hero needs to actually survive long enough to get anything out of his abilities - and that's the problem.
Before 6.82 you probably could have gone for Mek. With the increased mana cost it starts to be a bit too costly. However, the shiny new Crimson Guard is generally a great item for Bristleback as he is one of the few heroes who build Vanguard regularly, and the active ability makes him and his allies even tankier. As usual though, the problem is actually getting there. Rod of Atos is another good option, giving you a slow that's more quickly accessible, on top of a good HP and intelligence boost. Basically it allows you to catch up and drown your enemy in goo afterwards, keeping them slowed indefinitely. Unfortunately all of these items come with a rather hefty price for a support to afford in any decent time. Bristle does have decent farming abilities once he gains a few levels (and intelligence), so if there's space for him to farm, then some of these items might become online in sensible time.
Blade Mail is another interesting item as it kind of turns your lack of tankiness against your enemies. They can still burst you down, but take non-trivial amount of damage doing so. Should they choose not to, you get to stack some goo and quills on them before the spikes run out. It's overall a fairly good value item for Bristle too as it gives decent armor and intelligence, plus some damage. If you want, you can tank up with a couple of Bracers before going Blade Mail - in a way following the trendy three Null Talismans + Blade Mail Nature's Prophet build. If you expect to do something early, it might actually be best to go full-on Bracer gaming mode. It's probably the build I will try first. As a support Bristle is definitely greedy and has to be played as the four position, so forget about getting wards. You should still obviously carry TP scrolls.
Urn of Shadows and Medallion of Courage both have their merits. Bristle does have pretty high base mana regen after a few levels, so that 50% bonus can be very helpful. Both of these items also tank him up a bit with either HP from the urn or armor from the medallion. It still does seem better to have someone else build these items. It's also worth noting that medallion's primary purpose in the current meta is taking Roshan early and although building it would give you even more minus armor, you can already get -8 from nasal goo.
Since jungling doesn't look like an option, your life as a support Bristle should start by laning - and pulling. Unlike core Bristle, getting goo very early should be a good idea. Maybe even get the first point at level 1, as you won't be needing those quills anytime soon (possibly you could clear the pullthrough camp using it - so just spare the point and see what's needed). Either way, by level 2 having one point in each active seems like the obvious way to go. Whether you should then go for more goo or quills is another question entirely. One interesting prospect of level three goo is the ability to take Roshan. The doubled minus armor in general is quite strong if your team has other heroes with high early game physical damage. Taking the single value point of your passive should still be a good idea. The two possible builds by level 5 would then be 3-1-1 or 1-3-1. Having both active abilities leveled up by six also allows you to stack Warpath faster.
I don't really know if there's much of a plan for playing support Bristle. Carry TP scrolls and go wherever there's a fight I guess. If a farming spot is free, go ahead and make good use of it. It's probably best to play in a lineup where none of your cores are using the ancients to accelerate their farm as you can make use of them in your downtime. It's probably best to build according to your GPM. A couple of Bracers might go a long way in helping you get some of that precious kill bounty by staying alive in fights. Still, I guess the biggest issue with support Bristleback is that you can't really play the hero properly. Conceptually, he is in fights to be in the enemy's face and soak up damage, which is something you really cannot do as a support with no real items. As a support you will still do good damage in prolonged fights, but someone else needs to take the damage. Well, at least you are one tanky support if you're good at turning your back.
I only played this once, but it does feel a lot like I expected. Thanks to his high int growth, Bristle doesn't need any mana items to fight. He does need HP though, which can be helped a lot by grabbing a couple of bracers and treads. So it's not an entirely impossible idea to play Bristle without that much farm - after all, he is often played in the offlane anyway. He just doesn't offer much as a support, so he's very much the equal of very greedy supports like Elder Titan. He does have one interesting prospect in lane though. I suspected his defensive capabilities, but nasal goo is in fact quite strong in discouraging the enemy from attempting any plays against your carry. First, it's cheap to cast, and if one instance is enough to drive the enemy away, you can do it all day. Second, they really should turn because allowing Bristle to stack even level 1 goo two or three times makes them very susceptible to being turned around on.
It's important to remember that once you get goo'd twice or thrice, you can be goo'd almost indefinitely - and the minus armor really hurts! So, at least support Bristle is a somewhat efficient dive deterrent. You still can't really harass in lane, nor can you gank with any sort of efficiency. You also cannot do your job as the frontline in battle, because in order to be a threat you need to stay alive - which makes Bristle the very opposite of support Wraith King. Although, yes, you could soak up some damage, having less levels in Bristleback and even less HP from items, it just doesn't buy that much time. Especially since I think it's really important to go 3-1-1 as support Bristleback to help out your team the most. If you are ahead and winning fights, the chasing potential with this build is ridiculous. In general, support Bristle is fine when your team's ahead, but his comeback potential is abysmal - which in a way is an inherent property of the hero in general.
I stand by my earlier judgement of "poor".