It’s a compound of tathā (thus) + gata (gone) + ka (secondary suffix to indicate action noun) + [e]va (just).
In English, such suffixes are for example “er” as in “goer”. Here, however, it is added to gata, the past participle of gacchati, hence would literally be “goner”. But that has a rather different meaning in English!
The “o” indicates it is declined as nominative singular, and it thus agrees with the so as the subject of the sentence, i.e. the bird. Here the verb hoti indicates that the term is a predicate of the subject, i.e. “it (the bird) is a thus-goer”.
By the way, there are a couple of variant readings here, given in the DN passage: tathā pakkantova and tathāpakkanto va. But these don’t change the meaning; kanta is just the past participle of kamati, which is a synonym of gacchati.
This leaves the particle eva, which usually has the sense “only, just”. Here the meaning is clarified by the context. In the previous sentence, the bird flies (gacchati) to each direction. In the next sentence, it flies back (paccāgacchati) if it can’t see land. So in the middle sentence it must have the sense, “only goes there (and doesn’t come back)”.
This sense is not really captured in the translation you give, although the overall meaning is clear enough. Ven Bodhi has:
If it saw land anywhere, it went straight for it.
Here he seems to be echoing eva through the use of “straight”. While idiomatic enough, it is not very precise. To go straight there is to go without deviating. But what is meant here is that one only goes there. I would suggest:
If it saw land on any side, it went there and stayed.
Incidentally, in the sentence samantā is used, which is the ablative of samanta, “all around”. While the usage here is typically idiomatic for this term and thus the different translations express this in different ways, I think “on any side” is a more exact rendering.
Commentary has:
Sāmantāti avidūre. Samantātipi pāṭho, samantatoti attho
“on any side” means “in the distance”; the text reads samantā (= more ambiguous form of ablative), the meaning is samantato (= more explicit form of ablative).
Oh, and to forestall the obvious, no this doesn’t have anything to do with tathāgata as an epithet of the Buddha. The terms in this compound are extremely common and occur in a wide variety of senses. As an epithet of the Buddha, the primary sense is “one who has realized the truth”.