[This entry is taken from a chapter of Part II of the open-source textbook Spanish-English Cognates: An Unconventional Introduction to Spanish Linguistics.]
Fungi: Eng. fungus and Sp. hongo
So, a fungus is a member of the Fungi family of living organisms which includes as disparate things as table mushrooms and fungi that cause diseases or mycoses, as fungal diseases are called. The two words, fungus and fungi, are of course related, since in Latin, they are the singular and plural nominative wordforms of the same lexeme. Eng. fungus is a loanword from Lat. fŭngus, pronounced [ˈfʊŋ.ɡʊs̠], whose main meanings in classical Latin were ‘mushroom, toadstool, or related plant, fur or mildew formed on the wick of a lamp, disease of the olive tree, fool, blockhead’ (Oxford English Dictionary). The original form of this word in early Latin was sfungus and it is ‘probably a loanword from a non-Indo-European language’ (OED), perhaps ‘a Mediterranean–Pontic Pre-Greek substrate loanword’ (Wiktionary). The Latin word is cognate with, and may have even come from, the Greek word σπόγγος (spóngos) ‘sponge’, related to Eng. sponge and Sp. esponja, which clearly had a different meaning. English borrowed the word fungus from Latin in the 16th century, originally with the meaning ‘a mushroom or toadstool’ (OED), but later it came to be used in biology in the way we have seen.
The word fungi in English, the name of the animal kingdom, comes from the plural (nominative) of the Latin word fungus, namely fungī, pronounced [ˈfʊŋ.ɡiː] in Latin. This, Fungi, is also the technical or scientific Latin name of the domain to which fungi belong. In English, the word fungi has several possible pronunciations, such as [ˈfʌŋ.ɡaɪ̯] and [ˈfʌɲ.ʤaɪ̯], but also [ˈfʌŋ.ɡi], all of them reasonable spelling pronunciations. Note that this plural word has also been regularized by some authors to funguses [ˈfʌŋ.ɡəs.əs]. Linnaeus, the great naturalist, is the one who in 1753 coined the term Fungi for a class of plants. However, in the second half of the 20th century, these organisms came to be reanalyzed as a separate kingdom from the plant and animal ones. There are many different types of fungi, as we saw in the definitions above, namely (some) molds, rusts, mildews, smuts, mushrooms, and yeasts. We will look at those terms in turn in later sections.
The Latin word fungus is one of those that did get passed on uninterruptedly (orally) from classical Latin to vulgar Latin to Old Spanish and, finally, to modern Spanish. The word is attested in writing in the 13th century, both as fongo and hongo, which at the time was undoubtedly pronounced [ˈhoŋ.ɡo]. Its descendant, Modern Spanish hongo, pronounced [ˈoŋ.ɡo], means both ‘mushroom’ and ‘fungus’, in culinary, botanical, or biological terminologies. The sound changes from Lat. fungus to Sp. hongo are the expected ones, with (1) initial F‑ [f] changed to [h] in Old Spanish, which is written with a silent 〈h〉 today; and (2) short Latin Ŭ changing to o in Spanish as it invariantly did in patrimonial words (cf. Part I, Chapter 10).
The word hongo can be used with the same meaning as its English cognate fungus, in botany and medicine. In cuisine, but also in botany, however, hongo translates as mushroom, be it poisonous, edible, psychotropic, or medicinal. English has another word for ‘mushroom’, namely toadstool, though nowadays that word is rare or restricted to non-edible mushrooms or even to poisonous ones. The Encyclopedia Britannica equates toadstools with poisonous mushrooms, but other sources simply say that a toadstool is just another word for mushroom, ‘especially [though not necessarily]: a poisonous or inedible one as distinguished from an edible mushroom’ (MWC). The word mushroom is more common, though, for either edible or non-edible (poisonous) types. The word toadstool [ˈtʰoʊ̯d.stuɫ] is obviously a compound of the words toad and stool.
Spanish also has a second, additional word for the ‘mushroom’ sense, namely seta. In later sections we will discuss the words Eng. mushroom and Sp. seta. But before that, let us look at a few words derived from Lat. fungus in English and Spanish.
 The word mycosis [maɪˈkoʊsəs] (Sp. micosis) is a New Latin medical term formed in the late 19th century, from the root myc- of Ancient Greek μύκης (múkēs) ‘fungus’ and the Latinate suffix ‑ōsis, from Ancient Greek ‑ωσις (‑ōsis) ‘state, abnormal condition, or action’, an ending found in ‑όω (‑óō) stem verbs with the suffix ‑σις (-sis) which added to verb stems forms abstract nouns or nouns of action, result or process. The derived adjective is mycotic (Sp. micótico/a, de hongos).
 Eng. sponge and Sp. esponja come from Lat. spongĭa ‘a sponge and, by extension, things similar to a sponge’, which is a loanword from Ancient Greek σπογγιά (spongiá), which is derived from σπόγγος (spóngos) ‘a sponge; tonsil’, or else may have been borrowed by Latin from the same source that Greek got the word from. Sp. esponja is presumably a semi-learned word (DCEH). Eng. sponge [ˈspʌɲʤ] is attested in Old English (c. 1000), with the same spelling. Presumably the word was taken from Latin, not from a Romance language (OED).
Greek σπόγγος (spóngos) was borrowed by other languages, if not by Latin, such as Arabic: إسفنج (ʾisfanj), which passed it on to Persian: اسفنج (esfanj) and, from there, to Urdu: اسفنج (isfanj).
 As the OED explains, ‘Fungi were originally included in the plant kingdom, but have some features (including the heterotrophic mode of nutrition and the ability to synthesize chitin) shared with animals, and are now regarded as constituting a kingdom separate from both. Some of the organisms originally regarded as fungi, esp. lower fungi (see lower fungus n. at lower adj., n.1, and adv. Compounds 2b), are now classified as protoctists. Many fungi are saprobes, deriving nourishment from decaying organic matter and so playing an important role in the recycling of organic material in the environment; some are potentially disease-causing parasites of plants and animals, and others are symbionts, as in lichens and mycorrhizae.’ (OED). Cf. https://en.wikipedia.org/wiki/Fungus