Unlike Latin "ursus" or English "bear", the Slavic word "medved" is not cognate to the reconstructed PIE "*Hrtkos". It is believed that this word is a Slavic euphemism meaning "honey-eater". I am interested, however, what would happen if Slavs didn't use it, but still retained all the sound laws that shaped the Slavic languages. Can we try to reconstruct this word in Proto-Slavic?
Of course, the question is, which tribes are to be defined as 'Ancient Slavs'. In other words, into which depths of hystorical abyss we are to enter.
The meaning would be more like 'honey-knower', not 'honey-eater', because the Southern form of this word was ведмедь (and ведмід in more archaic Ukranian) and therefore in speakers' minds the structuring went most likely as мёд + вед.
Such a parsing is also confirmed by the Komi belives, whose language and culture preserved a lot from ancient Slavic times, seeing bears as spirits with a knowledge of wealth (the honey, alongside with furs, being one of the forms of ancient 'currency') and solar deities.
We should also mind the Paleoasiatic bear cult, common for many ancient tribes, not just for Slavs.
For Slavic languages, there are two options.
First, we can use the бер as in берлога (a place for a bear to lay) and бирюк (more often used to describe a wolf, but we shall return to the mistaking wolves and bears for each other later), but that did not derive from the PIE root you have mentioned, or even less common and, in my opinion, rather disputable ком.
Second, the Old Greek cognate is ἄρκτος (cf. the name of Arcas) which is related to Fenno-Ugric karhu, ohto and otso, as well as the Armenian [arjer] and Iranian cognates (Persian خرس [xarsh] and Kurdish hirç and hurç, Osetian арс [ars]).
Judging by the Fenno-Ugric parallels and exceptions from the I.U.R.K. law for Slavic languages, and also by the Iranian cognates, we have the initial [x], the following sound between [a] and [o], the [r] from retroflexive [d] and a final [s] with a shwa or two.
All these facts put together, we receive Х@р(ъ)съ [х@r(ъ)sъ] => Хърсъ [хъr(ъ)sъ] (cf. Ukrainian Khortytsa - remember the mistaking wolves for bears?) who was most likely a solar-bear deity than a loan from Persian theological system.