Why are horses and donkeys considered different species even though they can interbreed

Horses and donkeys are considered as different species because the offspring which is produced due to the interbreeding of both the animal called as Mule.

This Mule is sterile and cannot produce offspring.

According the definition of the species, the organisms belonging to the same species should be able to interbreed and produce a viable offspring which can further reproduce among themselves.

phylogenetic tree is the tree of life or evolution tree. it shows the different branches of evolution. the sequence goes like tissues - coelom - backbone - endoskeleton - segmentation