Sunic, let me get this straight: your awesome response to "buy whatever mount is appropriate to the campaign" is "well he could have bought a T-Rex, even though nothing even says T-Rexes would be available anywhere?" Really? First off, the T Rex can't fit most places anyway, so that's usually not a good idea. Second of all, if the DM actually allows you to buy Warbeast T-Rexes and has space for them, then you would have bought that and not the bat. Third, YOU went for the TO stuff (Warbeast T-Rex) while I was doing something that actually works in play (buy the best commonly available mount). Notice how the stuff I mentioned was all domesticated animals that should be purchasable (Hunting Bats are domesticated by the Desmoderu, the various Lizards are domesticated by the Drow, etc). And fourth, where are you buying this thing? The Warbeast rules are clearly there for purchasing animals that are actually domesticated purchasable animals anyway. Unless the DM has actually set up that there's a bunch of Halflings breeding T-Rexes somewhere, you can't just go buy one. But you could go buy a Quickpad Lizard (Raised by Drow) or a Dire Badger (Raised by Gnomes) or a Hunting Bat (Raised by Desmoderu) or whatever else is actually a commonly available war beast. Yes, the rules could be abused... but we're not doing that here.
The simple fact is, having an appropriate mount that can go where ever you're going is common sense, and not TO. Just because there's also a TO method for doing it doesn't change that fact, any more than the fact that Wizards can cast flowing time Genesis doesn't change the fact that they can also cast Glitterdust. One's overpowered TO stuff, the other's just a damn good idea. Hey, do you tell people they're also not allowed to ever buy magic items, because they could have bought a Candle of Invocation and thus the rules for buying magic items are too screwed up to be used?
And Warbeast isn't the only place with costs. Arms and Equipment Guide also has costs for various exotic mounts that are supposed to be available. I also mentioned the Draconic Cohort feat. One way or another, you could have a level appropriate domesticated mount... which is not the same as just assuming you can purchase a Warbeast T-Rex at level 5 in any given campaign.
So yes, it's perfectly reasonable to assume that an archer character can have a level appropriate mount with enough mobility to be useful. And as a result, Wind Wall is not actually a very good counter to them at all, due to its static nature. I'm sorry Sunic, but you just failed hard here. Try again.
JaronK