Yaks are just very hard to obtain through killing or making Summoning pouches. If you can get other animals, you can get yak babies. If killing they will be a small animal walking around you have to pick up. Its not uncommon to have to kill 3,000 to 6,000 or even more to get one. I have never got one through Summoning.
Unless you are an ironman and can't trade or just want to get them yourself, you can get them through trading other players. You can PM me if you need some, I have some baby yaks in my bank.Hi.