This is an extension on my article on mock castration in ponyplay. Here I will discuss chemical castration of a ponyboy using the drug Depo-Provera (medroxyprogesterone). This is not an article on how to do it yourself! You will need to consult a physician. Alternatively, you could go with some injection play using saline if you're just going for some of the psychological effects.
If, however, this type of feminization appeals to you, speak with your physician (preferably a physician who is BDSM friendly) about your goals and he will explain the possible side effects of the drug, and if he deems apprpriate, prescribe it to you. The drug is given by injection and will be administered by your physician. The drug is typically given once a month for as long as your physician deems appropriate.
In the interest of completeness, I will point out that U.S. "online pharmacies" will, after having you fill out a medical history form, have a licensed U.S. physician issue a prescription for Depo-Provera (medroxyprogesterone) and then ship the (oral form) of the drug to you. This is technically legal in many states, however, I strongly recommend against this route. If you talk with your primary care physician about what you want, odds are s/he will prescribe this or an equivalent drug to you. I realize this can be a difficult conversation to have, but I believe you will be much better off.
I know more than a few male ponies that identify as geldings, and, I know at least one male pony that identifies as a ponygirl, so I believe their is some interest in chemical castration.
Chemical castration is not really castration play, but more like feminization. Depo-Provera will cause feminization. It will decrease sexual drive, prevent erections, and enhance female characteristics. It needs to given on a regular basis to maintain these effects. While these effects can be reversible (to a degree) once it is no longer being administered, you should not assume that full functionality will return.
Although I've engaged in castration play on more than a few occasions, I don't have any first hand experience with chemical castration (usually my play is with an elastrtor and maybe some weights for good measure).
If this is definitely what you want, your physician can prescribe this for you (it is ultimately his medical judgement though - I am not a human medical doctor) and set you up on a schedule and proper dosing.
If this sounds intersting to you, but you're still unsure about going to your physician, I would encourage you to read more about chemical castration in BDSM, then talk with your physician (not your vet - they'll just cut your balls off) to find out the practicalities of undertaking this process.
If you are more interested in physical castration, then you might like my articles on castration play.
Alternatively, as I mentioned above, you might consider injecting saline and telling your pony you are giving him Depo-Provera (or perhaps putting a fake label on the syringe or bottle of saline and casually "letting" him see it). Don't underestimate the "placebo effect" - if your pony thinks he's being emasculated chemically, he may start acting more feminine!