Liddell and Scott say that ἄρκτος bear is feminine and all instances of the masculine are dubious. To me this is somewhat surprising. You would have thought that a ferocious animal like a bear would at least on some occasions be male. In the following passage where Odysseus sees the ghost of Hercules in the underworld, the boars and lions are masculine and the bears could be masculine or feminine.
σμερδαλέος δέ οἱ ἀμφὶ περὶ στήθεσσιν ἀορτὴρ
χρύσεος ἦν τελαμών, ἵνα θέσκελα ἔργα τέτυκτο,
ἄρκτοι τ᾽ ἀγρότεροί τε σύες χαροποί τε λέοντες,
ὑσμῖναί τε μάχαι τε φόνοι τ᾽ ἀνδροκτασίαι τε.
There was a dreadful strap around his breast, a golden belt where wonderful things were worked, bears and wild boars and fierce lions and fights and battles and murders and killing of men.
Odyssey 11 609