What does "isabel" mean? I couldn't find anything in any of the dictionaries i use. Book from 1907.

It’s a type of horse with a specific coat color; pale palomino or cremello, etc. The context of the sentence could have pointed you in that direction, as it is listing several other horse types/colors. Simply googling “Isabel horse” could have also brought you to the correct definition.

It's a type of whitish horse.  https://baroquehorsemagazine.com/history-of-the-isabella-horse/

From the context, it's clearly a type (or color) of pony... Which seems to be correct. But don't feel bad about not knowing that word. Id' never heard it before either.