I agree with both SynAge and Knight of the Lance - really;
read what I said.
I find it odd you would have a problem with someone not wanting to do these obvious jokes in the rules.
I never said that at all - I said
exactly the opposite.
I understand you are the kind of gamer who enjoys it, wants to do it, would have more fun playing the game, etc... That's awesome, and I respect that. Me? I'm an introvert who would have less fun by doing the same thing, therefore I would not. I'll still be having fun, and to be honest I would probably have more fun playing someone like you who is willing to do silly things like this, but it's not OK with me to actually expect everyone to do the same if they're not as willing.
But it's not about "having fun" - they are the rules. Now, certainly, they can be changed and discarded; but that is a conversation that happens. "Hey, mate - I really think the silly rules are, well, silly; can we just ignore them? Presume they always work?"
(As an aside, there are some that simply would not work for - I was just reading Greasus Goldtooth's rules, and you have to actually bribe your opponent (with pizza or something, I guess?) for his "make a unit not do anything" rule to function. So, having that work automatically wouldn't work - it would need to be a Bravery test or something. But that could be worked out).
And if someone said that, I would be "Sure!" because I understand. And I'm sure other people would be too.
But for someone just to say "That's not fun, so I'm not doing it" isn't fair - it's a change in the rules without agreement from the other player. It's not a douche action for someone to ask the rules to be played as written - especially if someone has chosen that model specifically! It's not like every unit needs a silly dance to move etc.
Again, this comes down to what I have been saying elsewhere - AoS is a game where people have to talk to each other, and come to agreements on what to field, what to play etc. And, also, whether or not to have the silly rules in place.
The point ISN'T saying "You absolutely MUST play these rules!" but rather "No, you don't get to unilaterally change the rules without consulting your opponent first".
Set aside the issue of talking to a horse; saying "I won't do this because it makes the game less fun" could be applied to anything. I won't remove my models because it makes the game less fun. I won't apply rending modifiers because it makes the game less fun. The rule about the imaginary horse is no less "real" than those. It is merely subjectively sillier.
There is a presumption of fair play when one is gaming - it is a presumption that players accept the rules and come to a mutual agreement over any changes. It's not about not talking to a horse - it's about the underlying unilateral dismissal of your opponent as someone you don't need to talk to.
Now, you've said you would speak with your opponent beforehand and say "let's not use those rules, okay?" But that wasn't the impression I got (before I wrote this post, which kept getting pushed back as more replies were made). As I have said (now) several times - I would be very open to someone saying "Let's change it up at bit" and wouldn't bat an eyelid at it. But I would want to be asked.