Shadowsandbag's idea is pretty solid. For each type of animal, you could say a number to roll for how many appear... 1 bear, 1d2 lions or tigers, 1d4 hawks or falcons, 1d6 wolves, 1d8 domestic cats or dogs, 1d10 rats, 1d12 bats, 1d20 tarantulas or scorpions, or a d100 swarm of insects. Also, make sure there is a different reason for the player to want to choose that animal over the others. If they are all just combat aides, everyone will just summon a bear or some lions. There should be good incentives to use them all. Consider choosing one special ability for each that gives it a completely unique reason to choose it, or ways that the player character can synergize with them.
For example, the bats could have an ability called Echolocation where the player can listen for their shrieks and use them to triangulate the location of an enemy in the dark, mitigating some of the penalty for low visibility.