"A scorpion" See the notes at Luke 10:19. Dr. Thomson (The Land and the Book, vol. i. p. 379) says: "There is no imaginable likeness between an egg and the ordinary black scorpion of this country, neither in color nor size, nor, when the tail is extended, in shape; but old writers speak of a "white" scorpion, and such a one, with the tail folded up, as in specimens of fossil trilobites, would not look unlike a small egg. Perhaps the contrast, however, refers only to the different properties of the egg and the scorpion, which is sufficiently emphatic."
Pliny ("N. H.," xi. 25) says that in Judea the scorpions are about the size of an egg, and not unlike one in shape.

Offer him a scorpion? - Σκορπιον. The Greek etymologists derive the name from σκορπιζειν τον ιον, scattering the poison. But is there any similitude between a scorpion and an egg, that the one might be given and taken in place of the other? We know there is the utmost similitude between some fish, especially those of the eel kind, and serpents: and that there are stones exactly similar to bread in their appearance; from which we may conjecture that our Lord intended to convey the same idea of similitude between an egg and a scorpion. Perhaps the word scorpion here may be used for any kind of serpent that proceeds from an egg, or the word egg may be understood: the common snake is oviparous; it brings forth a number of eggs, out of which the young ones are hatched. If he asks an egg, will he, for one that might nourish him, give him that of a serpent. But Bochart states, that the body of a scorpion is like to an egg, especially if it be a white scorpion; which sort Nicander, Aelian, Avicenna, and others, maintain to be the first species. Nor do scorpions differ much in size from an egg in Judea, if we may credit what the monks of Messua say, that there are about Jerusalem, and through all Syria, great scorpions, etc. Hieroz. l. iv. cap. xxix. col. 641, edit. 1692. To this it may be said, there may be such a similitude, between a white scorpion and an egg, if the legs and tail of the former be taken away; but how there can be a resemblance any other way, I know not. It is, however, a fact, that the alligator and crocodile come from eggs; two of those lie now before me, scarcely so large as the egg of the goose, longer, but not so thick. Now, suppose reference be made to one such egg, in which the young crocodile is hatched, and is ready to burst from its enclosure, would any father give such an egg to a hungry child? No. If the child asked an egg, he would not, instead of a proper one, give him that of the crocodile or the alligator, in which the young serpent was hatched, and from which it was just ready to be separated.

Or if he shall ask an egg, will he offer him a scorpion.... Of which there are three sorts; some are terrestrial, or land scorpions, scorpions of the earth, a kind of serpents, very venomous and mischievous, to whom the wicked Jews are compared, Ezekiel 2:6 and the locusts in Revelation 9:3 others are airy, or flying scorpions, a sort of fowl; and others are sea scorpions; of the fish kind: it is not easy to say which of them is here meant. There is an herb which is called (n), "the scorpion": it leaves are like unto a scorpion, as the Jewish commentators say (o). This is observed with the same view as the former. By it may be meant here, either the fish that is so called, since a fish is mentioned before; or rather, the land scorpion, which is of the serpent kind; this brings forth little worms, in the form of eggs, as (p) Pliny says: and it is said, that a scorpion put into an empty eggshell, has been used to be given to persons, whose death has been desired; which it bursting from, at once strikes and kills: but what father would do so to a child!
(n) Misn. Erubin, c. 2. sect. 6. (o) Maimon. & Bartonora in lb, (p) Lib. 11. c. 25.