@ the above: Not saying in any universe that Donphan's better than Mamoswine, but I don't think the two are comparable. Donphan's only niche is being able to set up rocks and spin at the same time. Spinning is obviously more crucial than setting up rocks (because far more pokemon get rocks than rapid spin), and the only pokemon Donphan might be comparable to in UU is forretress.
That being said, Donphan's specific typing and superior offensive attributes gives it a small niche on teams that sometimes forretress/blastoise/tentacruel can't achieve.
As for Feraligatr, I don't think dropping it is even a question. It has so few counters in the tier and almost nothing in the tier can firmly counter both sets (except maybe phdef p2/chesnaught/tangrowth, which I still wouldn't call "hard" counters), and even with proper checks either set can be devastating. I don't think there is almost ever a game where I use feraligatr and think that it has 0 possiblity to sweep; that's the great thing about Gatr, no matter how hard you prepare for it, it will always be threatening to some extent.
And finally, I'm tired of hearing the argument that suicune is a good gatr counter. Getting 2hko'd by SD after rocks and simply roaring gatr out is terrible. If you scald after switching in on an SD, you face the possibility of being 2hko'd (or being forced to rest if you live 2 +2 crunches). If you roar Gatr out and tank the +2 crunch, you get no chance to burn it, and if you roar in something that can OHKO a 50-ish% suicune (Rotom-c, Helio, Tyrantrum, Nidoking, Rotom-h, Azelf, Chandelure, Heracross, Shaymin, Mienshao, Whims, Dragalge, Abomasnow, Ampharos, Kyurem, Sceptile, Yanmega, etc) then you're forced to either sack your suicune or switch out. If you sack your suicune after roaring into something that can finish Cune off, you're probably going to lose to gatr later in the game (assuming that's your best counter). If you save Suicune, you come back in at 50% on +2 gatr later on the game and die to a crunch anyway.
And of course, there's also the option of going for scald on the +2 gatr and losing your "counter" 70% of the time. Either way, although Suicune may be a reliable counter to DD gatr, it doesn't counter gatr as a whole. SD gatr finds many opportunities in a game to get past Suicune.
Of course, a team with Tangrowth + Suicune would be well-equipped for gatr, but SD could still potentially sweep late-game (although not as likely). And if you're struggling so much with gatr that you need to run both tangrowth and Suicune, that shows how huge Gatr's impact is on the UU tier. The fact that Gatr puts this much pressure on teambuilding and either of its sets have colossal sweeping capabilities vs teams are why Gatr belongs in S-rank.