Chof Tammuz, 5774
In 5589, R' Yisochor Ber Horowitz, the Rov of Bichov Chadash (and later the Rov in Lubavitch), raised the question of a Jew who bought a cow from a non-Jew, along with a calf that felt attached to the cow and was nursing from it and that, strangely, had four non-cloven hooves - was it possible to permit such an animal for consumption? He first sent the question to R' Dovid Luria (the Radal), the Rov of Bichov Yoshon. The Radal initially ruled the calf forbidden, considering it an unusual mutation that could have been born from any animal and that was therefore assumed to have been born from one of the majority of the animals in the world, a non-kosher animal. However, in a follow-up letter, the Radal mentioned he was still a bit unsure as to whether the calf could be permitted.
The Tzemach Tzedek was later consulted and permitted the calf. However, due to the severity of eating non-kosher food, the Tzemach Tzedek felt it necessary to combine multiple permitting factors: (a) The extremely close resemblance to a calf makes it more likely to be the child of a cow, than of a non-kosher animal; especially since it was possible that even the calf's non-cloven hooves were not round like horse hooves, but rather shaped like cow hooves. (b) Though the Gemara in Bechoros (24a-b) questions whether nursing and a relationship between two such animals is proof of motherhood (and accordingly Shulchan Oruch (Yor"d 79:2) disregards such proof), nonetheless, in our case the fact that she is tending to it is partial proof of motherhood. (c) It's likely that the non-Jewish seller mentioned in passing that the cow gave birth to this calf, and in our case this can be combined with the other reasons to permit the calf.
(Sources: Tzemach Tzedek Piskei Dinim Yor"d 79; Yagdil Torah (NY) 9:41, 13:8, 13:12)