All good points here. I can only speak from the human female/male dog perspective because that's where my area of experience is but I can add some information. As the poster above notes, there are a multitude of reasons why knotting doesn't go the way we ideally think it should, from inexperience to the length of the dog's cock, and everything in between. Each encounter is it's own individual experience.
Someone above mentioned stretching or the colon, rather than the sphincter. And the same would apply to the vagina. The "goal" in using inflatables is really to simply familiarize the bitch with the sensation of inflation. Both the colon and the vagina are very elastic in nature, due to their individual functions. The vagina being by FAR the more elastic of the 2 simply because of what it is designed to do. However, the point is more or less the same in either case. I can't prove this next statement, but I firmly believe that ANY stretching you do of either location is NOT a permanent alteration of the space, but rather a need driven change that occurs as long as there is a need for that change. Simply put, if left unused for long enough, things will go back to whatever your normal is naturally if the extra capacity is left unused. And in BOTH cases, neither involves the need to stretch the OPENING. If you need to stretch the opening, you're doing it wrong. VERY wrong. And, you should stop immediately and figure your shit out because this is an ER visit that you don't want, waiting to happen. Filled with many unpleasantly personal questions you don't want to be asked.
Some people feel pain or unpleasantness during knot inflation (swelling), some do not. It's an individual situation rather than a general symptom. It's common, but not an absolute. Inflatable dildos and bulbs, and so on are a TOOL to train (I call it training) the future bitch what to expect. As I stated earlier, there *may* be an alteration of the space (which makes it PHYSICAL training), but the big picture outlook is a MENTAL training of the soon to be bitch. If you're doing this stuff on your own, I've already covered most if not all you need to know.
In my personal dealings, I ask each and every perspective bitch a series of questions designed to give me an idea of whether or not she's going to take to the act naturally or require some mental prep before she gets near my dog. All non-physical, mind you. I don't need to put my fingers inside her to know the answers I need, as long as she's honest in her responses. I do hammer home the importance of these answers with graphic detail as to what CAN happen if she's not telling me the truth. From ouch to 911. After that point, it's on her to be honest or tell me lies (it happens). If I get even a whiff of dishonesty from her, it's off. Period. Occasionally some bitch manages to skillfully deceive me, lucky for her, 99.5% (or more) of all pussy will accommodate dog knot without any lasting problems. So far, I've not run into any bitch who falls into the .5%. What happens is the bitch WILL feel some pain, to a lesser or greater degree and that's the end of it. My emergency measure is a glass of ice water (Yes, it works immediately). I have used the water, but I've never felt I HAD to, if that makes sense? When I have used it, it's been a CHOICE, not a reaction to events.
As for devices, ANY inflatable dildo, bulb, etc will work. And, if you have some cash and a taste for visual knottiness, Some company (never can remember the name...) sells an inflatable dog dildo named Kandahr that looks AND performs like a real dog cock. It comes in a large and a small. Almost everyone reading this should get the small. If you're a size queen, maybe the large, but I've never personally met a female who would NEED the large one, though I'm sure they are out there somewhere. It IS a good and effective tool. Highly recommended.