I am conducting a systematic review of animal studies, however, many articles are being excluded because only one treatment is performed. Ideally, systematic reviews must bring some kind of comparisons.
For non-randomized trials, you are advised to use ROBINS-I tool (Risk Of Bias In Non-randomized Studies - of Interventions) which is recently published by Bristol University:
However, it's more reasonable if you start reading about the latest version RoB-2 (Risk of Bias Tool 2), especially the guidance document to understand the concept of these tools:
What is your research question? It is best if you can formulate it in PICO format (see https://guides.lib.purdue.edu/c.php?g=352904&p=2378098). We can then better recommend a risk of bias assessment tool.