This is about female wanderers of other sects. There are a few mentions of non-Buddhist samanis in the EBTs, thus confirming that female asceticism was not uncommon at that time.
For me, this is also an indication that the Buddha was always planning to have a bhikkhuni sangha. After all, these women ascetics were already around. They would just come and listen to his teachings, just as the male ascetics, and since his teachings are so beautiful and convincing, how could they not become his followers? The Buddha didn’t really have a choice! 
Especially beautiful is the sutta with the very self-confident Samani Sucimukhi (“needle-mouth”), who challenges Sariputta at SN 28.10.
Here are the other instances where female ascetics are mentioned:
MN 45 simile of a female wanderer’s soft arm
MN 56 Upali closes his house to the Niganthas and Niganthis
AN 6.57 Female Ajivakas of Purana Kassapa
Ud 4.8 Samani Sundari is killed
Ud 2.6 The pregnant wife of a wanderer needs oil
Also, there are of course Vens. Bhadda Kapilani and Bhadda Kundalakesa, who were non-Buddhist ascetics before their conversion. They have their verses at Thig 107 (B. Kundalakesa) and Thig 65f (Bhadda Kapilani)