There are a huge diversity of hooks for terrestrials. Honestly, nearly any "trout-sized" hooks is a potential terrestrial hook. Terrestrial range from sub-#20 hooks for tiny beetles and ants to #6 2xl grasshoppers, particular for some of the larger western grasshoppers. Maybe the sweet spot for terrestrials is from about a #10 to a #18 but even within this range, there is a lot of variability. I could tie a stout, nearly round beetle on a standard or even short shank hook and an elongate beetle on a 3xl hook of the same size. Even for the most experienced of fly tyers, hooks are quite a black box mess.
This post is not exactly geared to the truly beginning tyer but the video above will at least get you somewhat acquainted with the terms and sizes I will use. And while we are talking basics, I do not think there is a tyer that does basic proportions better than Charlie Craven - if you want a little refresher on proper fly proportions, check out his 2013 article proportions.
What has generally made fly tying more complex than it needs to be is that there is no industry hook standardization to speak of. If you are looking for some clarity on hooks and how manufacturers label them, good luck with that! There are general trends but I would not say there is a standardization. One manufacturer's standard #18 dry fly hook may have a little longer shank, be of a slightly different wire gauge, or a slightly wider gape or bend shape than another. And the differences get even greater when you move to hooks different wire diameters (x-heavy or fine) and shank lengths (xl and xs). In general terms, a #12 hook that is 1xl, 1x-heavy has the shank length and wire diameter of a #10 hook but the bend size of a #12 hook.
It is not my goal in this post to talk about all the possible hooks, shapes, sizes, etc. but rather to specifically talk about common hooks for terrestrials. I will mostly use the Tiemco numbering system which has, I think, become pretty standard for most tyers and many charts to find equivalents from other manufacturers exist (here, here, and here to list a few). When I started tying, Mustad was the standard and when someone said 94840, tyers knew they were talking about their standard dry fly hook. Today, TMC 100 is that standard dry fly hook that other tyers know, though there are so many more manufacturers today that it has gotten a little more confusing.
Hooks for Terrestrial Flies
There are a few hooks designed specifically for terrestrials - most of which have longer and curved shanks, often made of finer wire and designed for grasshopper flies. Most of the time, tyers are using hooks designed for dry flies or nymphs and wet flies to tie their terrestrial patterns. There was once a time that the TMC 200R (3xl, straight eye, curved shank, standard wire) was a unique hook, now it seems most manufacturers produce a similar hook.
Dry Fly Hooks
Flies tend to float for two different reasons - they are less dense than water and/or they displace a lot of water and spread their mass over a large surface area. Like an aircraft carrier, you make make steel float. Certainly the easiest way to reduce the density of a fly is to reduce the weight of the hook. And the easiest way to spread out the weight over a great surface area is to increase the surface area (that is the kind of science you come to expect here..). Hackle, a wide foam or spun deer hair base, scruffy snowshoe hare foot fibers or CDC, a wide deer or elk hair wing, and many other common features of dry flies help them float.
Nearly any hook can be a "dry fly hook" with enough foam, hackle, or other things that help the fly float better. However, typically dry fly hooks are made on lighter wire gauges. And the standard hooks have a down eye, a straight shank, and a perfect (round) bend - this is the TMC100, 100B (black version of the 100), 100 BL (BL = barbless), 100SP-BL (SP = straight point), and 900BL. The TMC103BL is fairly similar but in black, on a lighter wire and the TMC101 has a straight eye, something that is nice for smaller flies, in particular. My favorite dry fly hook is the TMC102Y which has a sprout bend that puts the shank you can tie on farther over the hook point and is designed to better hook quick biting fishes. The TMC921 is a shorter shank hook that I do not believe I have tied on but would make for a good small ant and beetle hook. TMC's 5212 is a 2xl, 1xf (2 sizes longer, 1 wire gauge lighter) hook that is great for hoppers.
Following Ben's lead on his Ungamunga Hopper and Cricket, I have been tying the Morrish Hopper, probably my favorite hopper pattern, on Klinhamer hooks. The Tiemco model appears to be the TMC212Y but the standard here is the Partridge 15BN. I have generally tied them on the rather inexpensive Saber7018 Klinkhamer hooks. It is hardly a scientific experiment but the Klinkhamer hooks seem to do a better job of hooking trout compared to standard dry fly or nymph hooks. Part of it may be that the hook is fairly oversized so it has a larger gape which helps in hooking fishes - at least those that can fully get a hopper in their mouths. I also tie my Dr. J's X-Legs on a Klinkhammer hook and there are some really effective ants tied on these hooks as well.
Nymph Hooks
Nymph hooks are essentially the same as dry fly hooks but built from heavier wire. This is to get them to sink a little bit better and to allow them to pull free of snags more easily. It is maybe a bit counterintuitive to tie dry flies on heavier nymph hooks but I often do it on foam flies. A heavier hook will help the fly keel better - that allow the fly to land hook down. It may also help in that often important "splat" that draws fishes attention - probably because it is how larger hoppers and beetles land on the water. And because I am often fishing terrestrials in heavy cover and overhanging vegetation on heavy tippet, a hook I can pull out of streamside vegetation saves me from having to disturb water I would rather be fishing.
1xl shank length: TMC3761, TMC3761SP-BL
2xl shank length: TMC5262
3xl shank length: TMC5263
Any of these make good hooks for bulkier and/or foam flies. For small flies, like a smaller ant imitations, smaller beetles, and "my cricket"; I prefer a dry fly hook. For larger foam flies, if I am not tying them on a Klinkhamer hook, I am probably tying them on a nymph hook. And for grasshopper imitations, probably on the TMC3762 or TMC5262 or their equivalents.
Terrestrial Specific Hooks
Terrestrial specific hooks tend to be 2xl or 3xl curved shank hooks, often with straight rather than down eyes. Tiemco's three main hooks here are the TMC200R which is a very cool looking hook that I was once quite fond of but experienced hooking limitations on it. The flies tied on it do look very cool and natural - it was once my favorite nymph hook. Then there is the TMC2312 is a straight eye, 2xl, 1xf, hump-shanked hook that is a great terrestrial hook. The TMC2302 is similar but a slightly (standard) heavier wire and down eye version of that hook. Many other manufacturers' terrestrial hooks are 200R type hooks or 23XX variations but one that warrants a special mention is Umpqua's Stubby T hook.
Umpqua's "Stubby T" is officially a "Down Eye, 1/2X Heavy, Wide Gape, Forged Bend, Micro-Barb, V-Lock Bend, BN5X Finish" hook that is specifically for foam terrestrials. It is a great hook for smaller terrestrials - like Andrew Grillos' Hippie Stomper, a Driftless staple.
Wrapping it Up
Yeah, that was not EVERYTHING about hooks but it was quite a bit. I have no real brand loyalty to Tiemco, but I do think they make a great hook. But so do an ever increasing number of hook producers. I own and have tied on hooks from Ahrex, Daiichi, Dai-Riki, Firehole, Fulling Mill, Gamakatsu, Hanak, Hareline Core, Kona, Lightning Strike, Misfit Musky, Montana Fly Company, Mustad, Owner, Partridge, Saber, Tiemco, Umpqua, and Wholesale Fly Company I'm sure I've missed a few - like Targus which is no longer making hooks as best I know. I mostly write out that list and found their links so you can do you own research on hooks, their styles, prices, and such.
We are in the golden age of fly tying hooks. You could spend from about a nickel a hook to thirty or more cents a hook, quite a range of prices. I gave the models for Tiemco as TMC has become "the standard" but there are many different hook equivalent charts out there that a quick search by manufacturer will find plenty of options. Flyhooks.org is a website I ran across in researching this post and the images may be helpful in comparing hooks among manufacturers.
Hook Conversion Charts
Project Terrestrial Installments
Part 5 - All About Hooks (this post)
Part 6 - Sunken Terrestrials
Part 7 - A Terrestrial Fly Box