I've just tested the "Summon Kitty" and "Summon Puppy" items in creative mode.
Hearts appear and I can hear them but not see. They're invisible.

Not sure what is causing this?

I use the 1.10.2 version (SilentsGems-1.11.x-2.3.16-186)

edit: after a while and moving away from the place 1 of the kitty was slain. Short after all the pets became visible. After that spawning worked as intented.
Maybe a serverside problem.
Will close and reopen if the problem persists..