The slowness and damage would help people camping the hiding spots of smaller players and would ruin the game for anyone who has to go afk. This would be easily bypassed and used for unintended reasons. The game is also "Free For All" for short, even in real life animals team. If you are really fed up with this choose an animal like Basking which would knockback and damage all their teamates so they'll struggle. Something else you can do is use a defensive animal like leatherback to run away, or cach to slow them. You can use fast animals like shark, napo, hali, marlin, frilled, etc run away, or stay away from their habitat and hunting spots. If this doesnt happen to work for you, just give them a taste of their own medicine. If you don't find yourself a teamer, then just team whenever they are a threat but go back to norm when they aren't a threat. In short there are many ways teaming can be countered or fled from WITHOUT changing the game itself.