I was speculating the other day on the possibility that certain Indoeuropean terms meaning bear could have a Vascoid substrate as they resemble a lot the Basque word for bear: hartz. However I was soon corrected by a reader, Waggg, who indicated that there is strong evidence for a proto-Indoeuropean root *h₂ŕ̥tḱos and that variants of this word are widespread in Indoeuropean languages from both Europe and Asia.
I did accept the correction but my doubts remained: why of all animals only the word for bear appears to be an Indoeuropean loanword into Basque? Not even the name of the horse (the animal most characteristically associated with IEs) is such a borrowing (it is zaldi in Basque). Could it be that the word was introduced into proto-Indoeuropean before this linguistic family experienced expansion?
Another unclear issue is why the Basque word hartz has the /h/ at the beginning if it was a borrowing from Celtic arth? This initial h, which is silent in southern dialects but aspired in the North, is generally accepted to be a residue from an original /k/ (or /g/), which seems attested in ancient Aquitanian epigraphy. It would just make no sense whatsoever to have it added to this suspicious borrowing.
So I asked linguist Roslyn M. Frank (University of Iowa), who speaks Basque more fluently than I do, and she just replied attaching some notes with permission to publish. I reproduce them here in their totality (just boldfacing the last paragraph, which is maybe the most relevant one):
[Notice: minor corrections added in Nov. 28 by suggestion of author]
Unquestionably, the reverent attitude of these two bear keepers underlines the fact that the bear was deeply respected among the Basques. He was treated with similar reverence across both America and Eurasia in times past, as is evidenced in the case of rites for the dead bear celebrated until recently in Lapland, Alaska, British Columbia and Quebec. "All across North America, Indians have honored bears. When northern hunting tribes killed one, they spoke to its spirit, asking for its forgiveness. They treated the carcass reverently; among these tribes the ritual for a slain bear was more elaborate than that for any other food animal (Rockwell 1991:2)." As Shepard (Shepard and Sanders 1992: 80) has observed, there is evidence of a wide and ancient distribution of bear ritual. It is present in virtually every country of Western and Eastern Europe, in Asia south to Iran, and among many of the Indian nations of the United States, even into Central and South America. For example, the Asiatic Eskimos held that during the festival of the slain bear, the bear's shadow-soul could hear and understand the speech of humans and men, no matter where they were (Shepard and Sanders 1991: 86), while the Tlingit said, "People must always speak carefully of bear people since bears [no matter how far away] have the power to hear human speech. Even though a person murmurs a few careless words, the bear will take revenge" (Rockwell 1991: 64). The Basque bear keepers' words echo a similar belief in the bear's ability to understand human speech. And, far from describing him as a cuddly pet, the Basques' comments, represent the bear as a familiar yet awesome being, in a fashion comparable to that of northern peoples for whom he is "un animal intelligent, habile, humain, familier et redouté” (Mathieu 1984: 12).
Among Finno-Ugric peoples and Native American groups, the bear is viewed as omnipotent and omnipresent. He has the power to hear all that is said. For this reason hunters would avoid mentioning the bear's real name, choosing rather to address him with euphemisms. That these were the qualities attributed to the European Celestial Bear and his earthly representatives, can be demonstrated in social practice by the semantic taboo existing among Slavic and Germanic peoples which led them to avoid mentioning the bear's real name, an avoidance pattern which, in all likelihood, stemmed from a profound adherence to the tenets of this shamanic cosmovision. The substitute term utilized in Slavic languages was "honey-eater," while Germanic tribes preferred to call him the "brown one," an expression that gave rise eventually to the English word "bear," linked etymologically to the words "brown" and "bruin" (Praneuf 1989: 28–32).
In contrast, it would appear that other European peoples kept the original etymon for "bear", although they may well have avoided using it when they were in the presence of the animal or when they were hunting him. For example, there is the example of the name of the main character found in the Basque cycle of oral tales, Hartzkume. The word is a compound formed by (h)artz "bear" and -kume "offspring, baby." The set of Indo-European cognates for bear includes words such as ours (French), art (Irish), arth (Welsh), arz/ourz (Breton), arsa (Avestan) and rksa (Sanscrit) as well as arktos (Greek) (Buck 1988: 186). As is well known, from the Greek etynom arktos comes our word Arctic, the region lying towards the two Sky Bears (cf. Krupp 1991: 232).
Because of the phonological nature of the Basque word for bear, (h)artz (pronounced more or less like the second element in the English expression "fine arts"), linguists such as Holmer have argued that the word must be identified with the set of cognates found in languages classified as Indo-European. But in Basque, according to Holmer, the etymon is "conserved in a more archaic form than in any other Indo-European language (Holmer 1950: 403). In summary, the semantic relationship holding between the Basque word for bear and those found in the Indo-European languages cited above suggests that, in the case of this item, we are dealing with a wide spread archaic semantic artifact embedded still today in many IE languages. The IE etymon, because of its phonological similarity to the Basque item, could be traced back to a much older European linguistic substratum, one dating back to 4000 B.C., that is, to a period contemporary with bear ceremonialism and the point in time when the projection of the Bear Son stories skywards began to take place. Similarly, this chronology would situate the semantic artifact in Europe prior to the emergence of modern Indo-European languages, as a pre-Indo-European phenomenon that coincided with the rise of megalithic peoples and their fascination with the heavens.
 Specifically the IE etynom is bher-, "bright, brown," gave rise to the Old English form bera, and eventually to the Modern English word bear. The word "bruin" is a cognate of this group, often used in English to refer not to the color "brown" but to bears themselves (Cf. Watkins 1969: 1509).
 Praneuf (1989:28) cites the following western variants of the same etynom: Hindi rich, Gypsy rich, Persan khers, Kurd hirç, Sariqoli of Pamir yurkh, Pactau of Afganistan yaz, as well as two eastern European representatives that are remarkably similar phonologically to the Basque item (h)artz, namely, the Armenian arch or ardch and the form ars found in Caucasian, although the Caucasian form refers to a "bear cub" rather than to a "bear." Thus, further evidence for Holmer's hypothesis concerning the archaic nature of the Basque word is found in the similarity holding between the reflexes of the etynom in the eastern and western extremes of the geographical zone, e.g., in Armenian and Caucasian and in Basque. I would like to thank Ryan McGonigle who first brought the Armenian item to my attention several years ago. As an aside, it would appear that the Basque word began with an aspirated /h/. This sound has been lost in the southern dialects of the language.
Also she explained:
As for the 4000 BC time frame, that was simply a rough date, but one that IE linguists often assign to PIE.