Reconstructions of original Indo-European words are hypothetical and also educated guesses since the Proto-Indo-European language has been long extinct.
Both *Uhr-kto and *Orktho-s (Pokorny) have been suggested as the common ancestor for "bear (zool.) in many Indo-European languages: Old Irish, art, Welsh art, Albanian arí, Armenian arj, Greek arktos, Latin ursus (from an older orcsos) and Sanskrit rkshas. According to Vladimir Orel, the Albanian word seems to involved a feminine suffix -ina and he reconstructs the proto-Albanian word for bear as *arina.
In the Germanic languages, however, this word was replaced by a word meaning "the brown one" *barna or *bjarna and in the Slavic languages by a word which meant "honey-eater" e.g. Russian medved. One theory is that among ancient Germanic and Slavic hunters the bear eventually became a sacred animal so you never called him by his real name but by a euphamism instead: "the brown one"; honey-eater etc.
To know how linguists reconstruct words you also have to know something about Grimm’s law of consonant changes. This explains how English fire, Icelandic fyr, Greek pyr and Old Armenian hur can all be traced back to a common root for fire: * pa-uh-w-r as reconstructed by some linguists.. Where proto-Indo-European -p- changes to f- in Germanic and -h- in Armenian.
Because of Grimm’s law of consonant changes, some bizarre-looking words in other Indo-European languages turn out to be related to familiar English words as in the case of Gaelic olc "bad" and English ‘wolf’; Albanian fjal "word" and fshij "to sweep" with English ‘spell’ and ‘sweep’ or Russian dozhd/dozhdlivy "rain/rainy" with English ‘dust/dusty.’ :)
