Humping is purely for fun, it doesn't need to be more complex, the fox is already shooting his cummies on them.
I am open to thoughts on improve the tails though. I can't just read the horse is bad and not be suggest how to make it better.
To me at least, the big problem is that it has no range. It's basically just a melee attack that you have to charge up. The thing about melee attacks is that typically they're up close, snappy, and quick with an option to back out quickly, but when you have to charge them it takes away that control and makes it feel clunky and annoying. This basically just means that trying to use it is more frustrating than its worth and it makes you way more prone to getting hit. Out of all that tails, it easily places first in the lack of range, something that every other tail (except rat) has. The rat tail still works since it has a unique way of attacking that while needing you to be close, acts more as a protective layer and works in its own way. Not all the enemies really work fighting close up, because some can usually hit you before the charge up animation is finished. The tanuki and shark are the best examples of this, you just don't really have a good time to attack them with the horse tail.
Also I understand not wanting to put anything in for the hump animations since it doesn't really need anything to be worth seeing. It's just that once you see them once, there's not really any incentive to see it ever again. You can go the whole game without seeing any of them except for the shark, like how on my first time playing I didn't see the instructions and just thought there was some other prerequisite for seeing them. It doesn't need anything else to make it worthwhile but it also could just be more than what it is. One way it could be is it could be a time limited chance to get either a new tail, a power up, or a health up, just some health, or nothing at all. Kinda like a luck of the draw. The caveat to that being that there's only so much time before they go away and that other enemies can hit you while humping. That could give a bit of risk-reward to it that doesn't necessarily make it overpowered but can help newcomers and incentivize the mechanic for those who have already seen the animation already. Of course that's just an idea that might not fit your vision of the game and regardless I still do like it as it is, just that I feel like it’s missing a bit of oomph.
But what can I change with the horse tail? I know the weaknesses now, but that doesn't tell me how to make it better. I honestly have no idea how to change it. Keep it a melee attack, make it a projectile? how to make it different from the ones I have already? I don't have that information.
I'm kinda keeping the humping as is really. As I already said, it's purely for fun. Fun doesn't always mean you need to get something from your actions. Would it be better if I removed it?
Enemies do drop items, tied to tails, but very rarely. Right now it's a random tail. But I may make them exclusive to the enemies that have them. but that also means you can't get an otter tail in a level that contains no otters.
First off, there's a couple ways to fix it, but this isn't all of them. I also don't think you should change it entirely to make it a projectile like the others, uniqueness helps add more strategy to the whole game. A melee build has a lot of potential and even though I don't like it as it is, I do think it could be really good. Some ideas that you could do are; you could add a bit more reach to it, lower the charge time to it, give it an effect where it stays in place to hit the enemies a little bit after you fire it before it fades away, make it more of a risk reward type of deal where since you have to get so close it does more damage that other attacks, slightly increase rate of drops, or it could deflect shots back at the enemies if its timed right. it doesn't have to just be one of these either, you could mix and match to see what works.
Secondly, I actually did experience the enemies dropping items a couple times, and I don't really have a problem with any of that. It was merely a suggestion for an add on so you don't need to use the idea to change something that already works well enough, even if it seems a bit too rare for how short the game experience can be. I think the idea of tying tails to the animal that drops them would be cool, but that kinda plays against what animals are in it. Like I might be stupid but I don't think there's a wolf nor a horse. That would also bring to question why you can use enemy specific moves the enemies cant. Though I also think that would be a good reason to not give them away from enemies that don't have that tail. If you end up getting an otter drop from an otter in one of the few levels you can it'll probably feel rewarding, like a "yes, I deserve this" type of deal. Though that might not be the case if you make tails only drop from the enemy
And lastly, its your game, so whatever fits your vision to it is what should be there. If that's how you want the humping mechanic to work, by all means keep it that way. Just like its in your taste to have it as a fun little add on like it is right now, it is in my taste that there should be a bit more to it. That doesn't mean you have to change it or that you even should, but rather just a way to know what some people might be wanting out of your game. Part of the creative process for pretty much all things is knowing who and what your making it for. The game is a pretty good megaman like game with its own mechanics, but you also made it with the express intent of having its audience (you yourself are included in your audience) be gay furries who want something nsfw out of it. Otherwise there'd be no reason for it to be a a gay furry nsfw work game in the first place if that wasn't the audience you were making if for. Some people will be wanting more out of the humping animations, and some people like yourself won't. Its all about playing into what your ideal audience and you yourself want out of it. If that ideal audience and yourself don't want it then you shouldn't do it, but there will be people like myself that will come to this game hoping for a bit more there. That doesn't mean you have to cater to the audience like me, but not only should you be aware that an audience like me might exist, but that that audience has a right to be wanting something out out of any creative endeavor that ends up no being there. Which again, you don't have to play to. Me wanting more out of not only doesn't mean you have to/should add more to it, but it certainly doesn't mean you should take it away either. Sorry if this section comes across as catty and rude, looking through this again some of them feel that way.