This is where we'll be keeping a running list of rules that depart from the standard rules, as well as a compendium of rulings that will clarify anything ambiguous about the rules as written. Please note that these are IN ADDITION TO any setting-specific rules that supersede the core rules. For example, Tolrea departs from the "one step" rule for cleric/deity alignment matching. Consult the setting material for Tolrea for all of that stuff.
Mounts and Related Equipment
The Core Rules don't differentiate much regarding the nutritional requirements for various mounts. There's a single line-item for "Feed," costing 5 cp and weighing 10 lbs. All of the stock choices for mounts and beasts of burden (horses, donkeys, mules, and riding dogs) are mentioned in the descriptive text for this, but there's a huge difference of size between a Clydesdale and a Saint Bernard. Not to mention that one enjoys eating grain, and the other prefers meat. Probably this was just an oversight. The entry for trail rations shows that a day's food for a Small character weighs 1/4 as much as for a Medium character. And if we look at some of the spells and magical goods associated with keeping people fed and watered, we likewise see a distinction based on size: three Medium characters are roughly the equivalent of one horse (a Large creature).
I don't think any of us are really interested in getting far into the weeds on this, so I'm going to rule that a day's feed for a Medium mount still costs 5 cp, but it only weighs 3 lbs. This splits the difference between 1/4 and 1/3 the weight of the standard ration of animal feed. Riding dogs and other carnivorous mounts require a day's feed each day. If you do not have a ration of appropriate food for a carnivorous mount, you can use the "get along in the wild" application of the Survival skill to do without rations for the animal (and potentially yourself). But this reduces your overland speed by half, making it potentially disruptive if you have someplace to be and are in a hurry to get there.
We're playing a campaign that offers unusual mount choices. Goblins are well established as using worgs and wolves as mounts. A wolf mount may be obtained for the price of 150 gp. Such a mount possesses all of the stats associated with the "Wolf" entry in the Monsters section of the Core Rules, and is trained for the purpose of combat riding. Advanced wolves may have up to 6 Hit Dice; starting at 4 HD, they gain a size increase to Large. Advanced wolves may be obtained for a price of 200 gp x Total HD (so a 3-HD wolf costs 600 gp; a 6-HD wolf costs 1,200 gp). This may seem like a steep premium, but it's necessary because it's reflective of the advanced wolf's greater BAB, base saves, higher Str and Con, improved armor, extra feats, and higher skill points (all of which will be allocated by the DM, not the player). Purchasing an advanced wolf mount probably isn't cost-effective for you right now, but in future you may want to revisit the matter.
Worgs are effectively NPCs. Obtaining the services of a worg mount is most straightforwardly achieved via the Leadership feat, which will be available to anyone who meets the prerequisites starting at ECL 6th. Worgs start at 4 Hit Dice, and have a +1 level adjustment when used as cohorts, making them effectively a 5th-level cohort. Worg cohorts will advance by Hit Dice (and gain size to become Large at 7 HD) until they top out at 12 HD, after which time they stop improving. This is unlikely to happen anytime very soon; you have to reach VERY high levels to have an ECL 13 cohort. Be aware that worgs, like any other cohort, will expect to be rewarded for their service with a half-share of loot. And like any other cohort, they will expect you to provide for food and shelter, or failing that, to provide for enough downtime for them to see to their own needs.
Hiring a worg to serve as a mount is theoretically possible, but the barriers are many, as there is fierce competition for their services.