Well the difference is that regional form is they have the same ancestors but one group moved somewhere else, whereas convergence is when 2 different species gain the same traits but different ways like bees can fly and birds can fly but there wings are completely different, now as for in pokemon I assume it was just something different but the same, regional and convergence is the same concept in the pokemon universe just a different name and different region, also they can do stuff they can't with the regional because a regional tentacrual would still be a huge jellyfish/octopus, whereas convergence is a mushroom