Had Mari survived and developed her power, she would have been the first dog and first canine-based hero in Sentai.
The first to have a power connected to a canine (though not in his designation) was Saizou (NinjaBlue) of Ninja Sentai Kakuranger, whose mecha are all based around the wolf.This is also similar to the third Ninja base series of Shuriken Sentai Ninninger where Fuuka Igasaki (ShiroNinger) mecha the OtomoNin Wanmaru is based on the Japanese dog breed the Shiba Inu.
The first Sentai with a canine-based designation was Hikaru (Ginga Yellow) of Seijuu Sentai Gingaman; his powers and abilities likewise are based on the wolf. The power of the wolf would return with Tsukumaro Ogami (Gao Silver) of Hyakujuu Sentai Gaoranger.
The first dog-based Sentai hero was actually the dog-like alien Doggie Kruger (DekaMaster) of Tokusou Sentai Dekaranger; though his powers are not based around dogs, his species being doglike and his dog-like appearance in his suit and his wolf-themed sword makes him as such.