I think there are two ethical issues here:
1) Authorship. By most definitions, the work you did does not qualify you for authorship, because you did not have a significant hand in drafting/designing the research or the manuscript (that doesn't mean some labs wouldn't assign authorship in this case, but that if you read the fine print for authorship requirements for journals or professional societies, animal husbandry would not be sufficient). It seems clear you did not have a role in the manuscript because you didn't even realize your name was not included in the authors list or acknowledgments until now.
It would generally be good practice for the authors to at least include in acknowledgements somebody who did as much technical support as you did, and it would also be good practice for you to be offered to contribute to the manuscript and analysis to attain authorship privileges. You should not have been offered authorship as an incentive if the PI and other student did not intend to allow you to earn it.
2) Research ethics and appropriate use of graduate students. I think this area is where you have a greater case. It sounds like you were made to do way more work on another project than should be typical for a graduate student. A technician, sure, because your primary position would be as a paid assistant in the lab, but as a student, your primary position is as a student.
I am unclear why your PI thought this other student should not learn to work with the animals when their project was so dependent on animal work - that doesn't seem to be full training, how is the student going to start an independent research career if they have not learned the basic techniques of their field?
By no means should you expect to always work solely on your own project, but, unless you are overstating your contributions, which I understand is certainly possible, it seems like you were made to go beyond the normal level of assistance. I would consider carefully the technical contributions the primary author made to the project, and weigh how your efforts compared in terms of total hours.
Have you also had time to work on your own independent project, and is that work progressing to your satisfaction? If yes, then maybe there isn't a major issue, and this is just an unfortunate situation you will learn from in the future and you certainly now have a good reason to stop doing this work for other students. If no, then you certainly need to address this with your supervisor, and if you can not reach an arrangement that is suitable to you, you may need to talk to your program or department to get further guidance.