I cannot recommend specific works cause most I've read were in not english. Ethology is the field that studies animal behaviour and cognition. I'll brush up on a few thoughts, you can verify them if in doubt/interested. Another comment replied to you with exemples of painful events that can make one suffer badly: having your bodily autonomy violated especially is pretty traumatic whether you have language to rationalize it or not, for humans as for animals. That's because animals do have a sense of self - a lot more than a newborn human. Besides, animals worry for their future and try to prevent bad things from happening to them. There are also universal things that go beyond language, abstract thinking and society - think motherly love - and it can lead to huge amounts of suffering even without complex thoughts around it. Animals grieve, and allegedly may have committed suicide on occasion.

I understand the difference between pain and suffering you're meaning to convey, but animals are not a good exemple here.