As an image is often easier to understand as a long text. Here we go
0_1581122875470_FilterTrainDragon.png

Before fights you can easily filter dragons but you can't do that for training. It would be good to have filters on rarity, elements at least. A useful option would be also All trainable dragons / Only trainable dragons that are not fully trainable (can we train again a dragon that has four trained attacks?) And finally a filter on the dragon Rank. "A" ranked dragon are the one I mostly use so they are almost trained whereas no ranked dragon are almost not trained and that's between them I would like to find dragons to train.